Martin O'Neill learned "a big lesson" in terms of team selection after seeing a much-changed Aston Villa crash out of the Carling Cup to his old club Leicester.
A second-half goal from Matt Fryatt earned the Foxes - currently 21st in the Coca-Cola Championship - a shock 1-0 win at Villa Park in the third-round clash.
O'Neill made six changes from the team which comfortably disposed of Everton in the Premier League at the weekend. But only Shaun Maloney, of the players drafted in, emerged with much credit - to leave O'Neill contemplating whether he was right to revamp his team.
"Sometimes in the past I've changed the side around, and it has cost me," he said. "Maybe people should just deserve to be in the side - or be in the side when someone gets injured.
"I still wanted some players to play. But I think you have to earn the right to be in the team - and that's a big lesson.
"It reminds me of when I was at Celtic. We had beaten Liverpool on the Thursday in the quarter-final of the UEFA Cup. We went to Caledonian Thistle in the Scottish Cup; I made nine or 10 changes - and we lost the game."
O'Neill will not forget that - and it seems he may have another painful reminder of the dangers of squad rotation.
"That sat uncomfortably with me for a long, long time - and this would be the same," he said. "I am disappointed - but we should still have been good enough to win it and we should still have the same desire. That's the disappointing aspect."
Leicester boss Gary Megson questioned whether the players retained by O'Neill - and not rested like their team-mates - would be in the right frame of mind as his side pulled off a morale boosting win.
"We said in the dressing room before we went out, by virtue of the fact that they were actually playing and not being rested, that maybe one or two of the Villa team would look at it as being a little bit beneath them," he said.