Mourinho saying our summer window will be "short". So probably just 3 players, or if anyone leaves then replace. Assuming nobody leaves that we don't expect to (Romero might leave, but Pereira would just step up, Ibra and Carrick are expected to leave, and then Fellaini, Darmian and Blind will all probably leave).
Leaves us with:
GK: De Gea, Pereira
Back 4: Valencia, Fosu-Mensah, Bailly, Jones, Smalling, Lindelof, Rojo, Tuanzebe, Shaw, Young
Midfield 3: Pogba, Matic, Herrera, Pereira, McTominay
Front 3: Sanchez, Lukaku, Martial, Rashford, Mata, Lingard
Can potentially see Ozil coming in on a free, who would then make Mata pretty irrelevant, but again, that could go either way. Midfield the obvious weak point, missing the right balance for a midfield 3, and depth isn't good enough bar Herrera being Matic's back up. Seri, Kovacic, Fred and Milinkovic-Savic all been linked, would guess Seri the likeliest of those, though could well need one more if Andreas Pereira doesn't work out as a good squad option. Also I'd say we're still weak at fullback. Left back is risky to go into hoping Shaw is over the injuries and just having Young as back up, while right back Valencia is getting older so needs to start slowing down eventually, and Fosu-Mensah I'm not sure if he's quite good enough. Though maybe worth giving him a shot at being a squad player. Rojo can fill in at left back occasionally, lindelof at right back (and Tuanzebe even), so I don't think we'll be too desperate for anyone. Overall I'd say we'll definitely sign 1 midfielder, maybe 2 depending on McTominay/Pereira development for Mou, then if we can get Ozil on a free we'll get him, and then 1 fullback (either side, not sure).
Meunier, Kovacic, Seri and Ozil in would be a fantastic window, but we'd probably need to then sell Mata, and Pereira and McTominay would leave for loan/for good (not sure McTominay will ever be much more then squad player anyway, but Mourinho seems to like him).