United sit eighth in the Premier League, just two points outside the top four, but could have been in a far stronger position had they not also failed to capitalize on leads against Tottenham and Nottingham Forest in recent weeks.
Amorim's men also lost 1-0 at home to Everton despite playing against 10 men for almost the entire match in their previous game at Old Trafford.
"We should have closed the game with the ball because the game was there to win," said Amorim.
"The game was under control. We knew it (we needed to) defend far from our box as any set-pieces can be a problem.
"We couldn't do it, we couldn't maintain the ball and again we lost two points."
Diogo Dalot opened the scoring for United just before the hour mark, but Amorim cut a visibly frustrated figure after Soungoutou Magassa levelled from a corner seven minutes from time.
"I think the overall performance was not perfect. We had our moments but we lost control in the game in some minutes in the first half and in the second half, especially after the goal," added Amorim.
"It is really frustrating because you look at the game, you have the game under control and we didn't win."
