For Owen Hargreaves, it is difficult to imagine his comeback could have gone any better. His career has endured more miseries and been written off more times than he will care to remember but, finally, there is clear evidence to suggest he is at the point of re-establishing himself as a formidable opponent.

Hargreaves, 30, did not merely outdo expectations, lasting 57 minutes of this third-round Carling Cup tie when the original plan was to come off at half-time, but he announced his arrival as a Manchester City player with a moment that demonstrated why his new employers have decided he can play a significant part in their campaign to catch, then overhaul, Europe's elite clubs. The opening goal was a peach and it came from his right boot, at a distance when not every player would have had a belief in their ability to score.

Hargreaves was undoubtedly helped by the fact Birmingham were poor opponents. Nonetheless, he played with a confidence that made it seem almost a trick of the mind that at Manchester United there was a genuine feeling that the lights were about to turn out on his career.

"He's a fantastic footballer and he can become an important player for us," Roberto Mancini, the City manager, said afterwards. "It wasn't easy for him [psychologically] after so many years out, but he played well and scored a fantastic goal. If he can carry on playing often, training often, and become the player he was, it can be important for the England team too."

The road back has been long and tortuous but Hargreaves looked sharp, bright and as quick into the tackle as ever. Most importantly, he looked fit. When he left the pitch it was to a standing ovation and, before that moment, the home supporters audibly made the point that Sir Alex Ferguson may come to regret releasing him.

A long way to go until then, of course, but the mood was understandably light on a night when Mancini's experimental team had little problem moving into the fourth round and Hargreaves was not alone in terms of reflecting on a satisfying evening's work. Here, too, was Kolo Touré's first match since his six-month ban for taking a prohibited substance came to an end.

Alongside him Stefan Savic, the £6m signing from Partizan Belgrade, had a capable first start. Abdul Razak, a 19-year-old from Ivory Coast, patrolled the centre of midfield before succumbing to cramp. Wayne Bridge was in from the wilderness to play as a left-winger, with another full-back, Pablo Zabaleta, on the opposite side. Nedum Onuoha's presence at right-back emphasised that the Garry Cook affair will not be held against him.

With so many changes, it was probably inevitable that this would not be a performance totally in keeping with the slick and fluid football that Mancini's first-choice team have put together so far this season.

There was also the suspicion at times that Mario Balotelli and particularly Carlos Tevez were not going at full pelt. Balotelli, however, did take his goal with clinical precision, angling a left-foot drive into the bottom corner after Aleksandar Kolarov's low centre.

Two-nil up at the interval, the second half was a mere formality and Birmingham were left to reflect on the moment, shortly before Balotelli's strike, when City's debutant goalkeeper, Costel Pantilimon, made a hash of dealing with a free-kick and Curtis Davies's overhead kick was cleared off the line. Pantimilon, a Romania international on loan from Timisoara, is 6ft 9in – except for crosses, it seems, when he looks 5ft 8in. But that was really the sum total of Birmingham's challenge and their tepid efforts made it the perfect evening for Hargreaves to take his first steps back.

His goal was a beauty, too. Hargreaves has always been a sweet striker of the ball and this was a wonderful connection, arrowing his right-foot shot past Colin Doyle. It was a moderate goal celebration, as if he knew there were still challenges to overcome, but it still felt like a moment of great significance.

Birmingham looked defeated from that moment and by the end Mancini had brought on a 16-year-old, Karim Rekik, followed by the 20-year-old Luca Scapuzzi.

In between, the brooding Tevez ran clear only for Doyle to thwart him. Tevez's body language was all wrong but, for another former United player, it was a much more profitable evening.