That doesn't make sense, there are different registration rules for domestic leagues and European competitions. In other words clubs can decide to not register certain player(s) like how Chelsea left Palmer (and some other starters AFAIK) out of the Conference league group stage because they knew they didn't need him to go through against that level of competition.
We are also outside official registration window so can't really (un)register players unless they get uspended for gambling (ala Tonali) or doping but ofc that's a totally different thing than Thebes FFP shenanigans.
We can get around this by having him wear the number 38, Olmedo jersey.