The midfielder cost United £27m last summer but failed to win over some United supporters as he started just 12 Premier League games for the club.
But Fellaini has been a mainstay of the Belgium side which takes on Argentina in the quarter-finals of the World Cup tomorrow, scoring in his country's opening game against Algeria and playing every minute since.
Asked whether he had proved a point with his performances in Brazil, Fellaini told Sky Sports News: "I don't have to prove (anything) - I've been a professional footballer for eight years."
And questioned on whether he would still be at Old Trafford next season, he said: "We will see after the World Cup."
Both Belgium and Argentina have won every game on their way to the last eight, although the South Americans will start as slight favourites with Sky Bet - largely because they have four-times Ballon d'Or winner Lionel Messi in their side.
Fellaini said: "Everyone knows Messi is the best player, but we have good players as well. We play all together and we will try to play well on Saturday."