It's a difficult one, assuming José wants roughly 25-26 man squad again (maybe a tad larger including a few younger players). This is first-team squad from this season:
de Gea, Romero, J.Pereira
Valencia, Young, Smalling, Bailly, Jones, Rojo, Lindelof, Shaw, Darmian, Blind
Pogba, Matic, Herrera, Fellaini, Carrick, McTominay
Mata, Lingard
Lukaku, Ibrahimovic/Sanchez, Rashford, Martial
Again, will José want to try and keep a squad of even more players than 25 happy? You have to assume McTominay is not going on loan, so he is in the plans. Carrick is obviously gone, with Blind and Darmian extremely likely to go. Fellaini and Shaw are in that kind of bracket too, so if they all go, that's five removed leaving us with 20. However, are Fosu-Mensah, Tuanzebe, and Andreas Pereira coming back to play in the first-team squad? If so, that's three slots taken.
Unless we get rid of a player or two who are more 'regular' choices for us, it's unlikely we sign more than three new players. The centreback one is the real mystery as Rojo has barely featured after the injury, but recently signed a new long-term deal with the club. I don't see why he's tied down to 4- or 5-years if the deal was just to hopefully get more money for him, so I'd have to think he's staying. Bailly has gone from probably the mainstay to who knows what at this stage. Lindelof has only had a year and has definitely improved. Smalling is the name that gets thrown around the most, but I am baffled as to why. He always plays when fit. He's José's first pick. If anyone is likely to be sold in order for us to get a new CB, then it's got to be Jones.
Martial is probably the most likely to leave from the forwards, just because Lukaku is going to always play and there's too much competition for the left side of the attack.