The Hammers easily dispatched United 2-0 at London Stadium.
"It's sickening when you come in and that's the result," he told MUTV.
"It's so tough to take and it's difficult for all our fans as well who were unbelievable all game. Unfortunately we didn't send them back with three points."
McTominay added that injuries to a number of players, including Paul Pogba and Anthony Martial, was not an excuse for the loss.
"It's no excuse," he said. "We have quality all over the squad and obviously some of the main players who play every week are injured, but so be it.
"It's a chance for young players to come in and make their mark and in the future we have to look to them and hopefully they can get some more game time because we need them. It's not a time to drop your head."