Well if he only concedes one goal a game that's not too bad I suppose.
He's a young keeper, making his first Premier League start at the Emirates, his one mistake over the entire game (which wasn't even that bad) was that his positioning was a bid suspect for that goal, but he made himself as big as he could, and charged the ball down as quickly as possible after a mistake from Colo but Cazorla just found the perfect spot, and it was a good finish, and Alnwick didn't do a thing wrong for the rest of the game.