The Bees had the lead through Ethan Pinnock before United fought back to win 2-1 through goals from Alejandro Garnacho and Rasmus Hojlund.
Frank said afterwards: "I think a lot of things went right in the first half. It was a game of two halves. It was fairly even in the first half but I thought we were slightly on top. Then we go 1-0 up and go in at half-time with that result. We knew they'd come out strong. It's not good enough that we concede after two minutes in the second half. If they don't score there then we know always it becomes more difficult. That was the disappointing thing."
On United's response, he continued: "Of course I give them credit, there's always two teams on the pitch. I think we can do better, we have to play two good halves. If you want to win here you have to have two good halves. So the second half was disappointing. Credit to Manchester United, some of their players stepped up. I think Eriksen was running the show, definitely!
On Brentford's opener and the controversy over Matthijs de Ligt being off the pitch, the Dane also said: "I don't know exactly where De Ligt should have been. It's a corner kick routine that we have worked on and it paid off. We don't know if it would have helped if he stayed on the pitch. It is what it is. I always think if you flip it - Manchester United would have wanted to take the corner at the other end if one of our player were off the pitch. He was bleeding so I think it was good refereeing.
"I think we did a lot of things right in the first half, which was very impressive. Second half was disappointing because I know what these players are capable of. That's the frustrating part."