The Reds were sent crashing out by the League Two outfit, who won at Oakwell through Harry McKirdy, Jack Bridge and Nathan Thomas goals.
Stendel made seven changes to his side in a bid to give some of his early-season fringe players some money, but he was disappointed with what he saw.
It was a first defeat on home soil in 90 minutes since March 2017, though their impressive league record is still in tact.
And Stendel admits he did not see such a poor showing coming.
He said: “Nobody expected this performance from us today. In the end, Carlisle won 3-0 and it was deserved. It was hard for us today.
“I think the quality from the players was good enough to win this game but the quality from the team was not good enough.
“We missed a lot of things and we have three days to play Charlton so much better than today. Only then we have a chance to win this game.
“They need time to improve and okay, it’s better when we win the game but we have too much mistakes in the offensive.
“We lost a lot of easy balls and this is when you want it too much.”
The Reds will defend their home record in the league when the Addicks visit in the Championship on Saturday, with Lee Bowyer's side also losing to League Two opposition as Forest Green Rovers won on penalties.