"You cannot guarantee when he'll be back. I was with him and the physios two minutes ago and he's already working. But it always depends on the player," said the boss.
"It's a blow especially because we lost him before and we've been talking about the understanding between him and Robbie Keane, and him and Steven Gerrard.
"It's important not to put too much pressure on it and for him to keep working.
"Clearly because he has been injured before it's a concern. We are working with the medical staff trying to analyse the reason why and trying to prevent it happening again.
"He was sharp, so it was a surprise. It was the end of the game. We just need to do exercises and check the balance of the player."