Ibrahimovic, 35, joined United on a free transfer from Paris Saint-Germain and has scored 17 goals in 28 Premier League appearances to answer critics who doubted he could still have such an impact given his age.
Fellaini was given the opportunity to get to know him soon after his arrival at Old Trafford and is not surprised by the part he has played in United's success this season.
"We came back from holiday at the same time and four of us trained away from the rest of the squad for a week," Fellaini said. "I knew him like everyone else, what he could say, what people said about him, but I immediately liked him.
"He's completely different to what is said about him. Ibra, he's class. Now that he's approaching the end of his career, people start understanding he plays a role, but he plays a good role, and he's a great pro."