The PL max squad size is 25 players, not counting anyone 21 and under. The home grown quota is 8. Or to be more precise a max of 17 non-home grown players over the age of 21.
The easiest way to look at the situation is to count the number of non-home grown players over the age of 21, then add or subtract likely transfer moves. When you get over 17, you have a problem.
If you consider that Schweinsteiger's contract is up in 2018. Zlatan is unlikely to stay beyond 2018. One or more of Darmian, Fellaini, Mata likely to leave before the start of 2018/9 season. We really don't have a problem regardless of whether Luke Shaw stays or goes.
Looking into the future, home-grown qualified: Pogba, Smalling, Jones, Lingard, Rashford. Then TFM a possibility, Pereira if he returns, similarly Januzaj, CBJ, James Wilson. Numerous 3rd keeper options that qualify as home-grown. You have to remember that the 24th and 25th players in the squad rarely feature under a manager like Mourinho.