CM is the first position to address - Anderson should be the priority target. Failing that, Baleba but for me, no to Wharton. James Garner has played as well as any midfielder in the PL this season, he has really developed into a fine player. However, his recently signed contract and standing with the Everton fanbase and club make a return to Old Trafford rather unlikely.
Some reliable depth at LB is needed as Amass and Leon are probably not going to be ready and relying on Shaw for one game a week is already a bit of a risk. Dorgu is clearly better up the pitch and must be regarded as such and not, as some still say, a LB option. Mazraoui and Dalot are options to fill in for Shaw but they are also the main players for the RB role.
CB is going to come down to whether the club are confident enough to move forward developing Heaven and Yoro alongside Martinez, or if they will entrust de Ligt who, like others in the squad, has injury concerns.
Radek Vítek could well become the reserve keeper and the extended contract and form he has shown at Bristol City and in Austria last season, prove he is certainly worth consideration over Bayindir.
If Zirkzee moves on, which would not be a surprise, then another forward will be required. Will an academy striker step up? Or perhaps a not so expensive reserve striker to provide more depth.
CM is certainly the main issue, with Casemiro leaving it becomes especially so.