I'm not personally seeing this transformative impact that some people seem to, he's a good player and will continue to be, Arsenal won the game with a jammy deflection, with City looking a bit inept without a few key players(Arsenal had some missing too, of course).
If they win the league, he will be deemed worth it, regardless of whether he's majorly responsible for it, or not. I still think they could have spent half the money on a decent DM and then tried to get more goals into the team, but they're doing ok on that front as is, to be fair.