In terms of depth, I always look at it like a 1st 11 and 2nd 11. If you can put out 2 starting 11s, then have a few versatile youngsters fill out the rest, I'd say you're in good shape. If you have more than that, overly bloated. Less, and you're struggling.
With United right now its:
Martial Rashford
Lingard
Pogba McTominay
Matic
Shaw Lindelof Smalling Young
De Gea
James Lukaku Mata
Pereira ???
Fred
??? Jones Bailly Dalot
Romero
Lee Grant, Angel Gomes, Mason Greenwood, Tahith Chong, Axel Tuanzebe, James Garner, Marcos Rojo, Matteo Darmian, Alexis Sanchez rounding out the squad.
Basically for clear gaps in squad, we need a starting RB so Young and Dalot can be back up for both fullbacks and we need a CM so McTominay can go into 2nd 11 (or rotate with Matic).
For bloated areas, Sanchez has no point, Rojo has no point, Darmian has no point. Gomes, Chong, Greenwood and Tuanzebe all have 2 lineups ahead of them taking minutes over them without even looking at Sanchez, but since Sanchez isn't leaving, then it has to he one of the front 3. Replace Lukaku with Sanchez without buying anyone and then our squad looks incredibly light in terms of anyone who can score somewhat regularly, so it leaves Mata as the player who was the easy option to get rid since his contract was expiring.
With Martial, Rashford, Lingard, Sanchez, Mata, Lukaku, Daniel James... theres just not enough space there for our youngsters to get the time they need. 2 of that group have to leave, but if one of that group is Lukaku, then we need to replace him, so it's only logical to get rid of the 2 that are crazy overpaid, past it, havent produced in years and we wouldnt even notice any hit if their minutes went directly to youth players, and the one had his contract expiring. Our squad would literally have been more balanced simply by releasing Mata. I guess in the scenario that we sell Pogba and Lukaku and don't buy any attacking players, yeah it's good to keep Mata. I'd say we will have much bigger issues in that scenario though.