The Belgium midfielder has struggled to hit the heights at Old Trafford that he enjoyed at Everton since his reported £26m move in the summer.
However, Neville, who was a teammate of Fellaini at Goodison Park last season, feels that he will eventually prove manager David Moyes right.
"The United fans haven't seen the best of Marouane Fellaini," he told the Manchester Evening News.
"What is important for Marouane now is to play well in a match that we win and he scores a goal. He needs to score and win. That is the next step for him.
"All the other things like that that go with playing for Man Utd, it takes time to get used to. As the games keep going on, he will just get better and better."
Fellaini could come up against his former club when they visit United on Wednesday.