Arsene Wenger said that his gamble to play Patrick Vieira against Valencia had backfired after his skipper went off injured.
Vieira, who passed a fitness test on a groin problem to start the game, was subbed after 37 minutes with an injury to another part of his groin.
The injury looks likely to rule him out of the game with Spurs on Saturday.
Wenger said: "Patrick was insistent on playing and felt good. Medically, he got the green light and I decided to give him a test on the pitch.
"I took the gamble to take him off if he didn't feel good in the warm-up but he was okay so that's all you can want.
"It's strange that he injured his leg on the other side but in exactly the same place and in the same muscle. Maybe he was over-protecting his leg."