Jose Mourinho described Eric Bailly as "naive" and admitted the defender will be a big miss in the Europa League final following his sending off against Celta Vigo.
Bailly and Celta's Facundo Roncaglia were both dismissed following a heated clash two minutes from time at Old Trafford, before United progressed 2-1 on aggregate following a 1-1 draw in the second leg of their semi-final.
The Ivory Coast international will now have to sit out the final against Ajax on May 24 due to suspension.
Speaking about Bailly, Mourinho told BT Sport: "I didn't see the incident, but he was phenomenal. The game was emotional until the end for everyone. Some kept control in better ways than others.
"I don't know but I think it was with Roncaglia, right? Roncaglia's not new in these kind of situations and probably Eric was a bit naive.
"We lose a very important player for the final and we don't have many."
Marouane Fellaini's first-half header doubled United's aggregate lead before Roncaglia set up a nervy final few minutes by nodding home for the Spanish side.
Bailly and the visitors' goalscorer were then dismissed before Celta striker John Guidetti wasted a golden chance in the final moments of the match.
Mourinho said he expected a close second leg after United failed to finish the tie during an impressive display in Spain last week.
"I could guess after the first match because it was similar to Anderlecht (in the quarter-finals)," he added. "You are the best team by far in the first leg, you have to kill everything in the first leg.
"Because we never score goals related to the chances we have and then we got an open game at home with all the pressure in our side. They gave us a very hard match so we had to suffer until the end. Everything was open until the last second.
"The boys didn't play a phenomenal match but they fought, they gave everything they had to give and I'm really pleased for them because 14 matches and we're in the final."