All clubs have to submit a squad of 25 players on September 1st, including 8 'home grown' players. This then can't be changed until January. If there are lots of injuries at a club then youth players can be introduced on top of the 25 registered players, but no more senior players can be registered.
Not really sure what to make of this rule. I guess it gives more chances to younger players and it stops the big clubs being able to dominate with their huge squads of talent that allows them to rotate every week. But I've read a few thing in the last few days that a lot of big name players may miss out on being registered due to injuries. Woodgate, King, Andy Johnson, Anderson, Hargreaves, etc are a few names who may be in that situation. This seems pretty negative in the those players have no incentive to get back to fitness, they'll basically be doing nothing for 4 or 5 months but still picking up their 40k, 50k, 60k+ a week.
