There's very few people currently playing for the club I'd definitely keep but the problems United face are both that there's very little value in the squad, and not being in Europe will hurt the club's ability to attract players. But y'know, that's their own fault, based on how poorly they've run the club.
I was discussing this on a discord this morning, and basically came to the conclusion that out of the current squad I'd really only try to keep:
That's a massive clear out that likely wouldn't happen over a single window, but probably should. Even then, United probably don't get great value for most of them (bar a theoretical Bruno or Rashford transfer). So United would have to be much smarter than they've been previously. One player they should bring in is Tosin Adarabioyo, who is out of contract at Fulham in the summer iirc. He's big, not bad on the ball and from Manchester, which matters for some people. Similarly, Danny Welbeck's an option - he has more goals this season at 33 than Antony, Martial and Sancho combined and is home grown, if that still matters.
I don't know how attractive a not-in-Europe rebuilding Man Utd will be to certain players, but if they can manage it, Bologna (4th in Serie A) have a couple of players that could very much improve them - Lewis Ferguson is a current international for Scotland with more than 200 professional league games under his belt in midfield at just 24 and has been impressive for them. Joshua Zirkzee isn't the player he was predicted to be when he first broke through at Bayern but he'd be a good facilitator-style forward who could link up play well. Whether they'd come from the successful Bologna to whatever it is United will be in the summer is a big question, especially as I imagine other teams will also be looking at these players.