Walcott tended to drift in and out of the match.
But Wenger told arsenal.com: "It is difficult physically to run at the pace of a 100m runner and give always the right ball at that pace.
"Also, it is difficult for a player to make 100m races 50 times. He is not a player who makes a burst of just five yards, he is a player who every time runs 30 or 40 metres, that takes a lot out of you, and when it comes to the situation when you have to cross the ball, he already has a big effort behind him. I do not know yet if he will start against Barcelona, because I have to study what I have to do. I have tomorrow and Monday, so I will have to see."