Nottingham Forest manager Aitor Karanka expressed his embarrassment and apologised to the club's supporters following the 3-2 Carabao Cup defeat at Burton Albion.
The Reds were out-fought at the Pirelli Stadium and the boss had no excuses or complaints with the end result in Staffordshire.
He said: "We had a chance to reach the quarter-finals for the first time in 25 years and it is a real missed opportunity. If we were complacent then that should never have been the case because we were playing to reach the quarter-finals.
"I'm really angry because a team from League One, a team which has played four games in ten days, has looked much better than us. They were much stronger and they wanted everything more than we did.
"I can't understand why that was the case and I don't want to say something that I'll regret tomorrow. I just feel embarrassed and sorry for the supporters who travelled this evening because we all came here thinking we could achieve something good.
"We knew that as the Championship side we were going to control the ball but we needed to do more when we had it."