Sanchez took out Mbeumo just four minutes and one second into the match to receive Chelsea’s earliest red card in the Premier League.
“It was rough… the red card [would be] difficult not only for Chelsea, it [would be] difficult for any club,” said Maresca.
“After the red card, all the planning and everything doesn’t exist any more.”
Maresca said afterwards he would have preferred to go a goal down than lose his goalkeeper.
“It’s the best solution, because we are still 95 minutes to play,” he said.
“I think even Robert is aware of that, but also it’s difficult because he has to take a decision in one second or two seconds.”
The Italian manager took off two attackers in Pedro Neto and Estevao Willian for replacement goalkeeper Filip Jorgensen and defender Tosin Adarabioyo after Sanchez was sent off.
The change effectively left Chelsea with a back five, three midfielders and just Joao Pedro in attack.
Maresca explained why he had made such a defensive reaction to the dismissal, saying: “We needed to defend with five players – we can defend with four when we are 11 v 11.”
However, that decision looked like a mistake when Fernandes opened the scoring and Palmer then went off injured.
When Andrey Santos replaced the England playmaker 21 minutes into the match it was the earliest any team in Premier League history had made their first three substitutions.
Chelsea failed to register a shot on target in the first half as Casemiro put United 2-0 up.
However, the Brazil midfielder’s ill-timed lunge just before the break left both sides down to 10 men.
Chalobah managed to pull a goal back in the 80th minute but it only proved to be a consolation for the Blues, who suffered a second successive defeat in what has been a tough week of away fixtures.
They drew at Brentford last weekend before losing to Bayern Munich in their Champions League opener on Wednesday.
“When it went to 10 a piece I felt no urgency from Chelsea to try to get back into it,” said former England striker Wayne Rooney on Match of the Day.
“They were slow, side to side. If you’re a Chelsea fan you’d want to see a lot more urgency.”
Saturday’s loss at Old Trafford leaves Chelsea sixth in the top-flight table, and they will hope to avoid a prolonged absence for star man Palmer.