Forest stunned United at Old Trafford to hand Amorim a second consecutive Premier League defeat.
The manager later said: "Tough game. We started really bad, the first play we suffered a goal and then it's hard to try to fight. We then controlled the game and had a lot of situations.
"We improved the movement in the last third. We managed to draw the game and then at half-time we were ready to go for the win and then started really bad. Two goals. Then we tried a lot of things with not a lot of quality.
"We didn't have many situations to score.
"Tough game in a tough moment but we have to continue the job.
"This is a long journey. We are improving in some aspects.
"We have to win games to help us improve the team.
"We are in the start of something so big that we pay attention to every detail. We already know it's a big job so we have to continue."