Dale were on a six-game unbeaten run going into this match but they were well beaten by Chris Wilder's men.
"There was a target of losing only four of these last 18 games and we've now lost a fifth game out of 17, so I'm disappointed with that," he said.
"We were a shadow of the way we have been, with and without the ball. And especially without the ball. But I don't want to be too critical of the players because we've come a long way.
"Yet one or two of them reverted to type. We've made one or two uncharacteristic mistakes. The players probably let themselves down and let the supporters down.
"For me there's no such thing as an end-of-season game but the player seemed to perform like they thought it was one. To me every game means something.
"But I also know, having been a player, that it's hard in games like this to put your body on the line when you might be out of contract and you could get a career-threatening injury by doing so.
"But overall I'm encouraged overall looking ahead to next season."