Amorim wasn't at the club when United sold Elanga to Nottingham Forest.
The Sweden winger will face his former club tonight and Amorim said: "It's completely different expectations at a completely different club. We can't compare players in different contexts with different playing styles.
"Right now Elanga is playing just behind the striker as a 'ten'. Sometimes coaches see something in a player that can help them develop. But again - it's a completely different club. You know the pressure we're under. Some players can adapt to it - others can't.
"We try to make decisions based on facts and what we see. Sometimes you see one thing and then the players go to another club where they can play in a different way. Sometimes you have done the right thing, but it can also go the other way."