United came from behind after Ethan Pinnock had put the Bees ahead to win thanks to goals from Alejandro Garnacho and Rasmus Hojlund.
Frank said afterwards, "It was a good first half, I was very happy with that, I thought we were the better team in that period of the game against a good team," he stated.
"We went 1-0 up, but it's so frustrating and annoying that they scored after two minutes of the second half - if they don't do that, you don't know what would have happened. But fair play to Manchester United.
"I thought that we were way too passive. But we are at Old Trafford, they have some top players.
"After the first goal, I felt like we lacked a bit more character. If we want to win here, we need more. Unfortunately, not enough of our players hit a high enough level in the second half."