The Bees thrashed United 4-0 on the day.
Frank later said: "I was very pleased, very proud of the performance. To beat Manchester United 4-0 - biggest club in the world, world-class players - makes some remarkable memories for the Brentford fans here. It's crazy. In some way unreal.
"It's no surprise that we win today because I know we're good. Maybe it wasn't a 4-0 game. We deserved to win, but it wasn't a 4-0. I really hope that people look into our performance and give us credit, because if you win 4-0 you must be doing something good."