The way I understand it is anyone who was 21 or younger at the start of January 2012 won't need to be registered for the 25 man squad next season so you can free up 6 places meaning the battle for squad places will only really be an issue domestically from 2013 onwards by which time it's quite conceivable that Scholes, Giggs, Berbatov and Park will all of left/retired
Anyone born in 1990 or earlier will have to registered in next season's 25 man squad. You need to be 20 or younger by 01 January 2012 in order to be registered outside the main squad for 2012/13.
The following players all have to be registered in next season's Premier League 25 man squad:
De Gea, Lindegaard, Amos
Evra, Fabio, Rafael, Evans, Vidic, Ferdinand, Smalling
Valencia, Nani, Park, Young, Cleverley, Giggs, Scholes, Carrick, Anderson, Fletcher
Rooney, Welbeck, Hernandez, Berbatov
That's 24 players. Jones is young enough to be registered outside the main PL squad but significantly he would have to be named on the Champions League 25 man A list because you have to have been at a club for two years before being eligible for the B list. You could of course sign a player and then leave someone out of the CL squad but I'm not sure we'd be comfortable with that scenario, especially if the player in question wasn't unavailable long term with an injury/illness.
Owen, De Laet, Bebe and PIG would all have to be registered in both the PL and CL squads but I've excluded them because they don't have a realistic chance of making the squad.
Macheda and all of the other current reserves and academy players don't have to be registered in the main PL squad next season. There's the odd player like Veseli and a few of the foreign first year academy players who wouldn't be eligible for the CL B list but everyone else will be.