The Bees were unable to take several good opportunities before Chelsea scored two late goals, booking their place in the Carabao Cup semi-finals.
"We gave nothing away and had the biggest chances of the game in the first half, and should have been in front, but in second half we struggled to create anything and were poor from set-pieces which is usually one of our strong points," Frank said.
"We need to do better with both goals, Pontus has had a fantastic season and will blame himself, and also the penalty – two minor mistakes cost us.
"I am really disappointed we missed a great opportunity against a top side.
"I went with the strongest I possibly could due to the Covid situation and used the bench in the second half to try and change the game.
"Up until they scored it looked a 0-0 game all over and heading for a penalty shoot-out. But in games against these top sides it is the smallest margins which dictate and they didn't go this way."
Need a VPN? Or seeking to switch? Then support Tribal Football by clicking here to sign up with our official partners NordVPN. Not only will you receive the best VPN experience, but also great discounts by signing up via Tribal Football. Thanks for your support!