The Reds boss said that he was frustrated by the nature of the 2-1 Championship loss on Saturday afternoon, admitting that his players were outfought on the day.
"I am disappointed with the manner of the defeat. We were not at the races today, man for man and collectively we were outfought and gave away a couple of very poor goals," Sky Sports News quotes him as saying.
"There were some half-hearted challenges to stop players and that has not been us this year, and that is probably the thing that disappoints me the most.
"But if you think I am disappointed, then the players are even more disappointed to have seen their unbeaten run ended. But that is the nature of this league, if you do not play well enough you come away with nothing, no matter where you are in the table."
The result moves Cardiff up to 12th place in the table, while Forest drop down to fifth.