The email dropped at 12.37pm on 13 September and it brought big news for Guardian employees. The company's policy, it read, is "no longer tenable". The house style guide – that great, immovable tome and friend of the insomniac – had long since decreed that capital letters ought not to take accents. There is now an exception. Mesut Özil's influence is far-reaching.

On the field at Arsenal, in the stands at Emirates Stadium, the £42.5m deadline-day signing from Real Madrid has changed everything. The weird thing is that in terms of his position, Özil was not really what the club appeared to need. Any supporter would have said that the summer priority had to be to strengthen in other areas, particularly at centre-forward. The manager, Arsène Wenger, had plenty of highly decent No10s, chief among them Jack Wilshere, England's great hope in World Cup year.

And yet Özil is precisely what Arsenal have needed: a grand declaration of ambition, a player to fire the dressing room and thrill the crowd; someone to remind them that a trip to the Emirates should be a fun way to spend a bit of free time, rather than a moaning ordeal.

The feelgood factor was pronounced on Tuesday night, more so than it had been for years as Özil put his stamp on the Champions League dismantling of Napoli. For 45 minutes, Arsenal made one of the best teams in Serie A look like fodder from the Championship and although the second half was more measured, the statement had been written and signed, complete with umlaut.

"It was a great message from the club [to sign Özil]," Mikel Arteta, the midfielder, said. "Everyone was expecting a top player – probably a striker, rather than a No10 – but at the end, we got an amazing player and he has lifted everyone. Not only us [the players] but also the crowd at the Emirates.

"You can see the people are excited again to come and watch us and he makes a big difference. And when you feel that, it is positive instead of everyone being nervy every time someone misses the ball. You feel it when the atmosphere is tense. When there is an atmosphere, you play in it, so it does affect you."

Özil's goal against Napoli, his first in Arsenal colours, was a thing of beauty. Aaron Ramsey's cutback from the right was bouncing but Özil cantered on to it, opened up his body and, with split-second timing, found the sweet spot of his left instep to send the ball, at pace, into the corner of the net. It was a moment that provoked awed appreciation and provided the latest rallying point for his team-mates.

Their revival was under way before Özil's arrival – the club's excellent form began in mid-March and Ramsey, for example, has seemingly improved each week since late January. The outpouring that followed the Aston Villa defeat on the opening day of the season betrayed underlying tensions. Özil has helped to smooth them away. Now there is conviction and bold rhetoric. "If we can keep everyone fit we are going to be real contenders in all four competitions," Wilshere said.

Players are growing, not least Mathieu Flamini, who rejoined the club as a free agent the week before Özil signed. There is something inspiring about his relish for the midfield dirty work. The striker Olivier Giroud has responded while Arteta, on only his second start of the season against Napoli, looked like a man in a hurry to remind everybody of his worth.

Özil has helped to take the competition for places, especially in midfield, to productive new levels. Wilshere was unable to get into the team against Napoli while Wenger still has Santi Cazorla, Alex Oxlade-Chamberlain, Abou Diaby, Theo Walcott and Lukas Podolski to return from injury. Nobody is guaranteed a starting place, even if it seems as though Özil should be.

"If you don't perform for one or two games you get dropped," Arteta said. "Competition is a great thing, it raises the level of training and that will raise the level of games. Özil gives you a step forward quality-wise and someone else on the pitch that can decide the game by himself. He proved again against Napoli that he is able to do it."

Özil's close control, flicks and deceptive speed were a feature but he is not overly decorative; rather, he is incisive and punishing. He has spoken about how the Turkish side of his heritage has given him the expression and technique while the German one brings the mental toughness. It is a heady cocktail.

Arsenal fans are delirious while at Madrid the cries of various players about Özil's loss have proved portentous. When somebody like him becomes available, you move first and ask questions later. His signing did not appear to be the product of a summer-long Arsenal strategy, more an opportunistic grab. Either way, the bounty already seems rich.