Every manager has his reasons for letting some of his players go to other teams either on loan or selling them permanently. In Antony's case, I think it was fair for Amorim to let him go to Real Betis instead of staying with Manchester United. It would not be good for Antony to stay because it would not help him develop anyway. La Liga might suit him better, but that doesn't mean Antony is not good enough to play in a Premier League club.
La Liga and the Premier League are indeed different, but this does not imply that one is far superior. Manchester United are in a slump and almost all the players are not in their best form, so there is no regret to be said even though some of the players who left will perform at their best with their new teams.
I agree that Amorim should let any player go who doesn't suit his style of play. Antony is expensive and it would be a waste to keep him in a club he has no future.
But Ruben Amorim should have said he needed to let him go for tactical reasons. He is claiming that the English Premier League is more physical. I also think that Antony is performing well because he is under an experienced and tactically sound coach Manuel Pellegrini.