Arsenal didn't play all that well yesterday. They still won. It's not an ideal situation, but as they say, it's a hell of a lot better than playing well and losing. Arsenal could not have won, of course, without Laurent Koscielny's late goal, but he wasn't the reason it happened. That honor falls to Petr Čech.
When a team plays badly, in order to win it usually needs one of two things. It needs a moment of offensive brilliance from an elite forward, or it needs the defense and/or goalkeeper to play brilliantly. Mesut Özil played well yesterday (of course, like you could expect anything else) but he wasn't able to conjure A Moment nonetheless. So it fell on Čech to save everyone's ass.
He spent a good portion cleaning up the mistakes of others. Here is one example, saving after a poor clearance by Aaron Ramsey. There were several other defensive mistakes that could have led to goals in front of a lesser keeper, but Čech was up to the task.
This – a save of the week nominee – probably also applies. I'm not sure how Georginio Wijnaldum got this open. Some falls to Mathieu Flamini, who didn't meet the standard set last week by Hollister Cal and doesn't do a lot to slow down Ayoze Pérez or stop his pass. Some falls to Koscielny and Per Mertesacker, who don't deal all that well with the run of Aleksandar Mitrović and thus leave Wijnaldum far too much space. But Čech's save itself is stellar, on par with what you'd expect from a player of his quality. He comes off his line exactly when he needs to. His slide allows him to stop a shot or an attempted pass to Mitrović, and he gets big as soon as it's clear Wijnaldum plans to shoot. The only way it could have been better is if he'd somehow managed to grab the shot, but hey, you can't ask too much of one man.
Top to bottom, Petr Čech simply was what we expected him to be yesterday. He was great. He won us three points.