The key that makes Man United players look more energised is because Amorim gives all players full confidence to appear on the pitch. All were given the same opportunity and told to compete to show their best skills. In contrast to Erik Ten Hag, who immediately treats players selectively, many of whom he backs up despite having good potential. So for the time being my 2 cent for Amorim.
You are right, when any new manager joins a fresh team, he will have to start afresh with his players, he doesn't care about the players performance under the former coach, all he cares for is how the players perform in the training, it will determine how he will set up his team that will favor him.
The players are now energetic indeed because all of them will be fighting for a starting spot in the team which is good because even players that the former coach has dropped might have a chance under the new manager if they play well in the training section.
If Amorim is able to recover Manchester United's performance soon, then Manchester United will still be aiming the favorite team everyone if given hope for because they are likely the biggest team in the competition.