One defensive lapse and a host of missed chances were the reasons the Rovers boss blamed for the early exit from the competition.
"We are in the middle of a bad run, but we played good football and created chance after chance," Souness said after the 1-1 draw at Ewood Park.
"We made one mistake defensively in the whole game and were punished.
"We should have scored five or six goals and had them on the back foot from the first whistle to the final whistle."
“We had people missing chances who would normally put those away.”