Fellaini left for the Red Devils in September 2013, having spent five seasons with the Blues following a then Club-record transfer from Standard Liege.
"I think Marouane has been very, very influential for Manchester United in the second half of the season," said Martinez.
"His past at Everton will never be forgotten, but, as a football club, we're very much concentrated on trying to get the three points and that's all that matters.
"I'm sure if there is a moment after the game to look back at what Marouane Fellaini did, then every single Evertonian will have positive memories of him. But, at the moment, all that matters is trying to win the game.