The South African midfielder suffered a groin injury during a training session in midweek.
Moyes said: “He hurt his groin on Wednesday in training and it looks as if he might be out for two to four weeks.
“Two weeks would be good, four would not be. It is a real blow because he played so well at Fulham and is an influential player.
“I’m hoping he can heal quickly and at least we have got the international break coming up.”
