The Belgian ended his five-year spell with the Toffees to move to Old Trafford in a £27m deal, but has so far struggled to win over the Red Devils fans.
Ahead of Fellaini facing his former club at Goodison Park on Sunday, Martinez praised his side for reacting well to losing one of their star assets and claimed that he would not get in the current Everton XI.
Furthermore, he stressed that he did not regret letting the midfielder leave, as it allowed him to return to Wigan Athletic to sign James McCarthy, who has been a key figure for the Merseysiders this season.
"I think you take a little bit of time to adapt to a new club and it's not easy," he told reporters. "Remember that it's always a different way of playing your football in many respects. I think that Marouane has always shown that he's a winner. He's a player who will find a way to be successful. That's what you want to believe with him.
"Football is not that straight forward that you bring a player from one environment to another and it works straight away. I think good players always find a way to be successful and sometimes it's a matter of time.
"But you look at McCarthy, Gareth Barry, Leon Osman - where would Fellaini have fit? In football you move on so quickly. I'm just happy with the squad that we have. And James McCarthy would never have happened if Marouane didn't go. I've just been proud of the squad. I have been proud of how we reacted in that moment. It could have been very difficult."
Everton could move back into the top four, and 13 points above United, by completing a Premier League double over David Moyes on his first return to Goodison Park since leaving the club in June 2013.