Fellaini scored 33 goals in 177 appearances during five years at Goodison Park before joining Manchester United in a £27.5million deal before the close of the last summer transfer window.
The Belgium international has struggled for form and fitness during a disappointing campaign for United as a whole and he is yet to score in 20 games.
His departure from Everton helped Martinez to bring in Gareth Barry and James McCarthy to improve his midfield, and Fellaini's old club have flourished this season.
Ahead of United's visit to Merseyside on Sunday afternoon, Martinez suggested it would be difficult for Fellaini to find a starting spot in his team, who are hoping to pip Arsenal to a UEFA Champions League place.
Martinez, who replaced United boss David Moyes at Everton last summer, said: "I think you take a little bit of time to adapt to a new club and it's not easy. 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.
"It's an incredible situation to be in with that mixture of experience and youth and everything. It's so natural. It looks like this side has been playing together for the last five years.
"You should never worry about losing a player because if Manchester United couldn't hold on to Cristiano Ronaldo, the rest of us have no chance."