Just like that, another Mario Balotelli myth has been debunked. Many people were sad to learn last month that the striker had never given £1,000 in cash to a homeless man, nor driven to the school of a young fan in order to confront a bully. But Milan's supporters were far more disappointed to discover on Sunday night that he was capable of missing a penalty after all.

Balotelli had converted the first 21 spot-kicks of his professional career with such ease as to fool some observers into thinking that he might be infallible. In Italy, reporters likened him to a machine – custom-built to convert such opportunities into goals. Several team-mates swore blind that they had never seen him miss – not once, not even in training.

They cannot have been paying very close attention. Away from the spotlight of competitive games, Balotelli has missed before. He might not do it as often as most other players do, but there was a good-sized audience on hand to see him thwack one effort off both posts during a session with the national team just a few short months ago.

Pepe Reina never got caught up in the hype. As Napoli prepared to face Milan last week, he studied Balotelli's tendencies just as he would any other opponent. The Partenopei have specific time set aside each week for such homework – with both the manager Rafael Benítez and the goalkeeping coach Xavi Valero joining Reina and his understudies to review film.

Together they watched Balotelli's previous penalties and discussed what they had seen. Perhaps they discovered something new, a tendency that nobody else had noticed. Or maybe they spotted the same trait that Giovanni Galli claimed to have identified earlier this month. The former Italy goalkeeper told Gazzetta dello Sport that you could tell which way Balotelli would shoot based on the speed of his run-up – but refused to divulge the details.

One way or another, Reina got things right at San Siro. Napoli were 2-0 up and on course for a famous victory when Balotelli stepped up to the mark in the 61st minute. The goalkeeper must have known that this was a critical moment. If he conceded now, then the home team would have plenty of time to seek out an equaliser.

For a second he seemed unprepared, adjusting his sleeve as Balotelli began to run forwards. But as the ball left the striker's boot, Reina plunged to his right, pushing it round the post with a single outstretched hand. It was a tremendous save – one that required not only an understanding of the attacker's tendencies, but also athleticism and strength to achieve.

Reina would later insist that he had been "lucky", but more than one Italian media outlet sought to present it as an "historic" save. If that was overdoing things a touch, then the goalkeeper's actions had at least helped to secure a historically significant result. Napoli would go on to win the game 2-1, claiming their first victory away to Milan in almost 27 years.

Before Sunday, their most recent success in this fixture had been the one that they achieved on 13 April, 1986. Milan's team that day included Paolo Rossi, Ray Wilkins and Mark Hateley; Napoli's featured a certain Diego Maradona. He scored one goal, and Bruno Giordano the other, in a 2-1 victory for the Partenopei – who finished the season in third place. A year later they would be crowned champions of Italy for the first time in their history.

Napoli's supporters hope that Sunday's result will prove the herald to a similarly glorious new era. Their team have made an impressive start to life so far under Benítez, with five wins from as many games across all competitions. The first three of those came against some of Serie A's lesser lights, but successive victories over Borussia Dortmund and Milan are rather harder to ignore.

Few had expected things to come together so quickly. Napoli invested more than £70m in upgrading their squad this summer, and the arrival of such players as Gonzalo Higuaín, José Maria Callejón and Dries Mertens undoubtedly captured fans' imaginations, but new signings often take time to bed in. These ones also had some large shoes to fill. Their transfers had been funded in large part by the sale of Edinson Cavani – who scored almost 40% of Napoli's league goals last season.

Even Napoli's returning players would be dealing with a significant tactical shift, as Benítez abandoned the three-man defence of his predecessor, Walter Mazzarri, in favour of a 4-2-3-1. A team which had built its success on caution and counter-attacks would now be asked to carry the game to its opponents and monopolise possession.

Such a transition cannot have been straightforward, and yet Napoli have made it look just so. Marek Hamsik, already one of the leaders on this team, has taken on an even more prominent role since Cavani's departure. His four goals in five games are testament to Benítez's insistence that he spend less time tracking back and more contributing to the team's attacks.

Callejón, Raúl Albiol and Reina have all slotted seamlessly into the starting XI, while Higuaín has begun to repay his £32.5m fee – scoring in each of the team's last four games. His goal on Sunday – a violent drive from outside the box to make it 2-0 – showed that he is capable of more than just getting on the end of a cross.

Napoli's players have praised Benítez's meticulous attention to detail, and yet he has also understood when to cut them a break. Following the team's win over Dortmund, he brought them in for a gentle exercise session on Thursday and then gave them Friday off.

Reporters covering the team were bewildered. Friday would have been Napoli's only full day of training before they travelled to Milan but Benítez felt that the additional rest would benefit his players more than any tactics session. The performance on Sunday proved him right, Napoli's energetic start catching Milan off guard as Miguel Britos opened the scoring in just the sixth minute.

They could not sustain such dominance, gradually surrendering space to their opponents as the game progressed, but the damage had been done. Napoli were grateful to Reina on more than one occasion – he finished the game with seven saves, and could do nothing about Balotelli's superlative consolation goal in second-half injury time – but fortune had favoured the bold. Benítez attacked Milan as Mazzarri had never dared to do at San Siro, and duly reaped the rewards.

Benítez recognised his own team's shortcomings afterwards, offering a delightfully precise assessment. "Before tonight we were playing at 72% of our potential," he said. "Now we're at 73%. I've seen how we worked at our training base and we can still get a lot better. And if we get better we can win a lot of games."

Five games into the manager's tenure, some would say that they're doing rather well in that regard already.

Talking Points

• Before we move on from this game, a thought for Balotelli, who played exceptionally well despite his penalty miss. Napoli never came to terms with the forward's movement, and besides being denied by Reina on more than one other occasion he also saw one second-half effort crash back off the crossbar. He did, however, then demonstrate his self-destructive side, talking his way into a post-game red card from the referee Luca Banti. Milan will miss him desperately during his forthcoming suspension. After four games, the Rossoneri are already eight points off first place.

• After Lazio defeated Roma at the Coppa Italia final back in May, a group of their fans held a mock funeral for their rivals – complete with coffin and grieving widow. When I mentioned the incident to Roma's new manager, Rudi Garcia, this summer, he replied with a shrug, saying "the fixture list has given us the opportunity to rise from the dead very quickly". So it proved on Sunday, Roma winning the derby 2-0 through goals from Federico Balzaretti (who promptly broke down in tears) and Adem Ljajic. After four games Roma sit level with Napoli in first place, having won all four of their league games and conceded only once. Not bad for a team that shipped 56 goals last year, and sold their best defender, Marquinhos, over the summer.

• Francesco Totti turns 37 on Friday, but despite the advanced years he was still the star of the derby – setting up Balzaretti's goal and generally sparking Roma to life after a slow first-half. He had signed a two-year contract extension on Friday, describing playing for the Giallorossi as a "never-ending dream".

• There were tears, too, at the Stadio Mapei, where Inter thrashed hapless Sassuolo 7-0 – the first time that the Nerazzurri had ever won by such a large margin away from home in Serie A. Diego Milito was unable to contain his emotions after returning from a seven-month injury lay-off to score twice. At 34-years-old it was no guarantee that he would be able to recover at all from the knee-ligament damage he sustained last February during a Europa League fixture against Cluj.

• Massimo Moratti might also have been excused if he felt like having a little cry on Sunday. The Inter owner has at last publicly confirmed his intention to sell the club to the Indonesian businessman Erick Thohir, and suggested on Thursday that the deal would be done before the end of the month – with just a few small details left to be resolved. Moratti will retain a minority stake – reported to be 30% – and is also understood to have written a number of clauses into the contract stipulating that the new owner must invest in the club and place its interests first at all times. If he fails to meet these requirements, the club could theoretically be restored to Moratti's control.

• Hello, at last, Fernando Llorente, who marked his first start for Juventus with a headed goal as the Bianconeri beat Verona 2-1. Amid growing speculation that the player was feeling excluded in Turin, this was exactly the outcome that both the player and his manager, Antonio Conte, will have been hoping for.

• Giuseppe Rossi scored his fifth goal in six games on Sunday as Fiorentina beat Atalanta 2-0. Not bad for a player still feeling his way back into the game after a two-year injury absence.

• Another man in exceptional form to start this campaign is Alessio Cerci. His third goal in four games sealed Torino's first win at Bologna's Stadio Dall'Ara in 33 years, and moved the Granata up to seventh place in the process.

Results: Atalanta 0-2 Fiorentina, Bologna 1-2 Torino, Cagliari 2-2 Sampdoria, Catania 0-0 Parma, Chievo 2-1 Udinese, Genoa 0-0 Livorno, Juventus 2-1 Verona, Milan 1-2 Napoli, Roma 2-0 Lazio, Sassuolo 0-7 Inter

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