We don't needany homegrown players. It's just that if we don't register homegrown players then we can't register anyone in their place as the squad space is reserved in effect.
Currently we have:
Foreign
Caballero, Sagna, Demichelis, Kolarov, Kompany, 'Mangala', Zabaleta, Fernandinho, Fernando, Garcia, Nasri, Navas, Silva, Toure, Aguero, Dzeko, Jovetic, Negredo - (18)
Homegrown
Hart, Wright, Boyata, Clichy, Richards, Milner, Lampard, Sinclair - (8 inc. 2 club trained)
Notable U21
Rekik, Nastasic, Guidetti + others
In the Premier League we are allowed at most 25 players over the age of 21, of which at least 8 must be homegrown (no club trained restriction). In the Champions League we are allowed at most 21 players in the A list, of which at least 5 must be homegrown, of which at least 1 must be club trained. A player is able to go on the B list if they are U21 and have been at the club for 2 years, which Nastasic now counts for.
So, in terms of foreign players, we're allowed to register 17 for the PL and 16 for the CL, hence you can take that at least one of the 18 above will leave and, if only one, another won't be registered for the CL. I'd imagine that Garcia will leave and either Negredo or Demichelis won't be registered - Negredo dependent on the severity of his injury and, if it isn't too bad, we have some decent B list CBs who can cover for MDM and don't need to be registered.
We're obviously trying to shift some of those HG players, but all can currently be registered for the PL. 5 can be registered for the CL (as I can't see us including any at the expense of a foreign player) and one must be either Boyata or Richards. Assuming Micah and Sinclair leave, I'd imagine Wright will be left out of the CL squad.
Any further buys (from the income from sales) will be difficult to wangle into the squad if they're either homegrown or foreign.