This is an analysis based only on the Prem squad rules. These always throw up interesting permutations and have to be a key factor in decisions about all fringe squad players. Last season, perhaps surprisingly given United's reputation for developing talent, there were only 9 'home-grown' players in our 25 man Prem squad and the minimum we must have is 8. They were Carrick, Johnstone, Jones, Keane, Lingard, Rooney, Schneiderlin, Smalling and Young.
None of the players added so far (Bailly, Zlatan, Mkhi) will be classified as home-grown though. If Pogba came back he would not be a home-grown player either as he just missed the cut-off. Also, some of last season's under 21 players step up into the full player age category (21 or over on January 1st, 2016). Memphis, Blackett and Love all fall into this category though of these only Memphis is not home-grown. Assuming that Memphis gets a chance of another year to develop at least that means we are adding at least 4 more players into the 25 who don't count as home-grown. Simple maths means that we have to lose at least three of the non-home-grown category over 21 players to make room as we can only drop one of the home-grown (from 9 down to 8).
For this reason, there has to be a lot of pressure on the places of people on the fringes of the team who are not home-grown: The likes of Varela and Romero are in this category, although Romero's place is slightly safer since Valdes left. There clearly wasn't space for the both of them long-term. Equally, it is hard to see Blackett or Love retaining a place unless more than one of the 9 'home-growns' mentioned above leave (as then they might get a free pass just to make up the 8 quota).
Thankfully, guys like Martial, Januzaj, Wilson, Shaw (amazingly!), McNair, Pereira and of course Rashford, CBJ and TFM are all still U-21 and so can stay in the squad without counting towards our 25 player quota.
By my reckoning then this makes Blackett and Love almost certainly on their way out (on loan at least) as there is no room for them. Valdes has gone already so the three other players to go are likely to be Varela (behind Darmian and Valencia and Bailly can play right back too) and probably Keane (on loan again as we have Zlat, Rashford, Martial, Wilson and even Rooney who can play 9) plus one from Rojo, Mata and Fellaini. If we have to make room for Pogba then two of them must go.
Rojo has to be a candidate with Shaw back, CBJ, Darmian and Blind capable of covering either at left back or centrally. I think Mata and Fellaini could go too but Herrera could be a surprise inclusion in the exodus if Pogba comes in.