The midfielder joined United from Everton in a £27.5million deal on transfer deadline day, but has failed to make the expected impact at Old Trafford.
“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.”