Fellaini, whose current contract expires in the summer, has missed United’s last five games with a meniscus knee injury having limped off during the 2-0 defeat to Tottenham Hotspur in January.
The 30-year-old underwent minor surgery on the problem last month and his manager expects him to return soon, but Mourinho is less confident about keeping hold of the player beyond the end of the season.
“His injury had a little surgery but minor surgery and he will be back to play again in a couple of weeks,” the United manager said.
“If we proceed in the Champions League after Sevilla I think he will be ready for a hypothetical quarter-final, a hypothetical FA Cup semi-final, for the last couple of months of our Premier League fight.”
On the question of Fellaini’s future, Mourinho said: “Is he going to stay or leave? I don’t know, I want him to stay, the club wants him to stay.
“The process, negotiations, numbers, distances between the offer and what he wants, I really don’t know.”
Fellaini is one of four players certain to miss United’s trip to Crystal Palace on Monday night.
Phil Jones (knock), Marcos Rojo (knock), Daley Blind (ankle) and Ander Herrera (hamstring) will also sit out of the visit to Selhurst Park, while Zlatan Ibrahimovic remains unavailable.
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