Mbeumo is returning to former club Brentford with United for today's early kickoff.
"If you look at Bryan, in those moments when we are struggling, you feel a different guy compared to the rest of the team," admitted Amorim.
"And that is, you have to understand, some of the guys are here for a while, and that feeling is still here. And for that, we need to win games. Simple.
"He went to pre-season in America, that was really good for us to bond with him in America. He had time to start slowly with 45 minutes, he had time to prepare the season.
"Again, I knew the player. I watched the player so many times. I watched him live against us last year, (though) it was not a good day for us."
Mbeumo live convinced me
Amorim recalled being impressed by Mbuemo when he led Brentford to their 4-3 win against United in May last season.
"It was not that performance," stated the Portuguese. "But (it) helped me to understand that he's a very good player, not (just) with the ball, without the ball. And when you see that live, it’s completely different.
"When you see a guy doing that, with all due respect, it's not because he’s in Manchester United. He was in Brentford, he was sometimes defending as a right-back. So, that was really important to have that feeling live.
"And if I'm the owner of Brentford, I will do the same, to keep the best players and try to sell the best players for big amounts. So, I completely understand that situation.”