The midfielder's future at Old Trafford appeared to be in doubt a few months ago after failing to make an impact under former boss David Moyes following his arrival last summer for a reported £27m fee.
However, the Belgium international has impressed in recent weeks by scoring an equaliser in United's 2-2 draw with West Bromwich Albion and played a vital role in his side's 1-1 draw with Chelsea last Sunday.
"He has made a great contribution and also the assist for the goal [against Chelsea]," Van Gaal told reporters. "He has had a lot of time out injured but when he is fit I have played him. He has to continue with that because then I am very pleased."
Fellaini could feature in this Sunday's Premier League clash against local rivals Manchester City at the Etihad Stadium.