Each PL club will have to name their 25 man squad by September 1st( new rule). This is the day after the transfer window closes. 8 of them will have to be home grown,meaning that they must have spent at least 3 years training with an English or Welsh professional club before they reached the age of 21. After that, the only players not on that list who can play will be any under the age of 21 on 1st January 2010. These new rules mean that a club can no longer be fined for fielding it's weakest team, any of the 25 can play. However, if too many players under 21 are chosen, then clubs could be fined. (Dont quite get that last line)
You can still play a player who is not in the squad IF he is under 21. If too many under 21s are used you could be fined ... because it could be considered to be a weakened team if you travel to Old Trafford with just the under 17s team and Derry!