Fermin has no business being in the u21. Logically he should have been in the Spain squad and the superstars of the Euros triumph rested. But of course sponsorship obligations probably require the big names to be called up.
He still gets injured for the main national team mind you.
Happens too often. A change of training system and then injured in training at the national team. You can't really counter it really.
But there should be an option for players to refuse calls for nations League citing rest and recovery. Official qualifiers, no.
Before the added competitiveness of nations League, national teams would test the waters more with call-ups for friendlies. A double edged sword has been created.