Don't think there will be as many as some posters think.
Reallistically I can see Pereira being made available, possibly going to a lesser league team, so won't get a lot for him, Dalot has done reasonably well at AC, think they may well plead poverty to get him cheaply, but I think he will leave.
Matic I think will want a good final deal and the chance to play most weeks, so can see him going, Roma has been mentioned, but that may just be the Jose connection, but Italy would suit him better I think.
Lingard also will also want to play more regularly, so can't see him coming back to United to sit on the bench.
Those are the definites I think, now of players who may go.
Pogba I would certainly make available, very talented player, but just don't see him fitting in anywhere in this current United team, and does disappear in some games, but, who would buy him I'm not sure, Real Madrid possibly? Which brings in a possible swap deal with Varane, if he indeed, wants to leave (always doubtful with players/agents using United as a tool to get pay rises), if that does somehow materialise, or we do get another CB, then I would allow Lindelof to leave, never thought he was quite up to Prem standard, gets bullied too often, and doesn't seem too great in aerial battles.
De Gea also I would put on the market, as I just see a gradual decline in his abilities over the next couple of seasons, is Henderson better than De Gea? Probably not at this moment in time, and is prone to odd mistakes (similar to De Gea when he first joined), but I can see Henderson improving in all departments over the next couple of seasons.
I also think Jones, James, Mata, Martial will be staying at least half a season, Jones will cover CB whilst Tuanzebe possibly gets a season loan, James will stay if Pogba goes, Mata is well liked by players, and may be one of those guys who is instrumental in helping younger players integrate into the first team environment, also helping on the coaching side. Martial I think will stay as cover for striker and left side.