In general we all know that injured players are remove automatically from NT squad.( even there is still time to recover before NT game!!!)
I don't know why you didn't write whole situation? He was injured yesterday, he is healthy today so he was healed or injury length was 1 day only. Yesterday game here
http://www.gokickoff.com/team_match_detail.php?match_id=8388290 and player Fenghui Ding
http://www.gokickoff.com/team_player_detail.php?player_id=6239417And the real problem is if he was automatically remove from U21 squad :
1) just after Custom Cup games
or
2) he was removed in daily update?
If 1) it is disaster cause any NT manager
can do nothing with that 
The only solution is to don't use NT players in custom cup games.
If 2) there is still time to heal player before update.
IMHO better way is to get human managers to decide if they want to have injured player or not w NT squad. It is simple but we all know GKO
