Pavel Nyakhaychyk is ineligible for EuroCup

hi I'm managing Galatasaray in Turkish Premier Division and I signed Pavel Nyakhaychyk.He can play in my league matches but I can't play him in Euro Cup.I joined Euro Cup starting from the 3rd round but when it was time to register my players for 3rd and 4th round it says ineligible.Do anyone knows why?

maybe he had already played a euro cup game for another team?

He must be cup tied, that or he just isn't registered.