The midfielder has failed to impress since his arrival at Old Trafford on transfer deadline day, but his boss says the Belgian will eventually round into form.
The Belgian joined the Red Devils for 27.5 million pounds on transfer deadline day but has struggled to settle into his best form, nine games into his Old Trafford career.
"He knows he can play better," Moyes said of Fellaini during a press conference. "When he came into the team, we were not playing as well as we would have hoped but, for a lot of people who join Manchester United, it doesn't always happen as quickly as you would like. He needed to take time but we threw him in very quickly.
"He also has a wrist injury that he will need an operation for and I think it's a little bit that he's protecting it. But he will be fine for us; we just have to give him a chance."
Fellaini spent five years at Everton under Moyes before following him to Old Trafford this summer.