Player Reg

Could someone please help with my problem. I am playing FM13 and am having trouble registering players for Champions League. I understood players under 21 do not need to be registered, so I ignore ones with Under 21 next to their name. However when selecting a team for a specific match, they are shown as “Not reg” Can you help?

It depends how long they've been at the club beforehand. I think the rule only applies if they've been at the club for two or more years previously, eg. Balotelli would be ineligible for Milan, as he has only just signed for them. Hope I helped