According to Calciomercato's Kaustubh Pandey, Rossoneri boss Gennaro Gattuso sees the Belgian as a player who fits his team terms of physicality and experience, but the club are yet to make contact with his representatives as they're concerned about his monetary his demands while they wait to hear from UEFA as to whether they will face financial sanctions or not.
Fellaini's contract is up in the summer and will be available for free, but his wages could still be an issue.
United have attempted to tie the 30-year-old down on a new deal, but he is yet to agree terms with the club.
He told Sport Voetbal magazine (h/t Charlotte Duncker of the Manchester Evening News):
"The entire staff is doing everything I can to stay. Now I am in a strong position, especially since Mourinho has stated that he wants to keep me.
"Last year I went to the coach and I said that I wanted a new contract. I then had a second meeting, but I will not ask it ten times. Afterwards I became important for the team. ... And a good player costs at least €50 million. The club knows that they went wrong there."
Samuel Luckhurst of the MEN and ESPN's Alex Shaw believe the Red Devils should allow him to leave:
While there's plenty Fellaini brings to the table, he's a limited player, and for a top-level side like United he has proved a hindrance to their ability to play attractive, expansive football.
He could be a perfect fit under Gattuso at Milan, though. The Italian has taken things back to basics at the San Siro this season, and there's scope for a physical, combative player in Fellaini's mould to shine as he offers a disruptive presence and aerial threat in the final third.
Squawka Football demonstrated what can make Fellaini such an effective weapon when utilised correctly:
Given his unique strengths it's understandable United have made efforts to keep him, but the limitations that come with him equally justify their reluctance to meet his demands.
Allowing him to move on would allow the team to do the same and could improve the quality of football on offer at Old Trafford, particularly if he's replaced with someone more gifted technically.
As for Milan, much will depend on the terms Fellaini wants, but he could be a useful acquisition.