Moyes confesses he cannot say when the Belgian midfielder will be back.
He said: "It is a worry because we are missing a really good player and someone who gives us a different attacking dimension.
"It wasn't a serious hamstring injury and we gave him a bit of extra time to recover. It wasn't as if we rushed him back.
"It may be scar tissue around the hamstring and he is getting seen to just now. I couldn't tell you how long he is going to be out.
"Around this time of year there are some players who come in from abroad and their bodies are used to having a winter break.
"That wasn't the reason he pulled his hamstring originally four weeks ago.
"But I do think adapting to the rigours of the Premier League could sometimes take a toll."