Dundee United manager Peter Houston felt his side "didn't turn up" after watching the Scottish Cup holders crash out against Motherwell.
He told BBC Scotland that the bruising 3-0 defeat at Fir Park was difficult to take.
"We had our second shot on goal in the 89th minute. We were second to every ball and the hungrier side won," Houston said.
"It was a poor performance and we've let a lot of fans down."
United, who drew 2-2 in the original quarter-final tie at Tannadice, never established any rhythm in the replay as goals from Jamie Murphy, Chris Humphrey and Francis Jeffers saw 'Well progress.
"They were hungrier, sharper, quicker to the ball and we stood off people," said Houston.
"Motherwell got in our faces and, in cup ties, you have to do that. On a surface like this (at Fir Park) you've got to use the ball and it's not the type of surface to be tippy-tappy.
"They played it like a cup tie and thoroughly deserved to go through: good luck to them."
The United boss apologised to the estimated 3,500 away fans who had travelled to the replay and he believed it was the poorest his side had performed since he became manager.
However, he dismissed concerns that his responsibilities with Scotland under boss Craig Levein was proving a distraction.
"Absolute nonsense," he said. "I missed two days with them. So, I don't care what people say and I'll continue to do that; I give 100% to Dundee United. They can say what they like but I'll be continuing to do work for Scotland."