Middlesbrough were beaten 2-1 by Blackburn Rovers yesterday evening.
Blackburn outplayed the North East side from the off, as they claimed a 2-1 victory in front of their own fans at Ewood Park.
And Karanka admits that he struggles to comprehend why his side seemed to show such a lack of motivation.
"It's really difficult for me to understand why a team like Blackburn, who is in the middle of the table and which had played five games in the last two weeks, and their players wanted it more than us,” the Spanish boss said.
"Their players are jumping more than us, they were quicker than us, even when they were winning 2-0 they were desiring to score the third one.
"When we are playing to get promotion, or at least to be in the play-offs, it is difficult to understand.”
Middlesbrough would have move to the top of the Championship table if they had managed to beat Blackburn, but they were unable to take their chance.
Rovers took a 2-0 lead after scoring through Hope Akpan and Danny Graham, and Boro did not respond until the moment that they were staring at such a deficit.
The in form Adam Forshaw did pull a goal back for Karanka’s side, but ultimately it was not enough, as they fell to a disheartening defeat.
Boro have now won just two of their last eight matches, and they are in danger of losing ground on the top two if they do not get back to winning ways quickly.
The Teesiders’ next match comes against Wolverhampton Wanderers on Friday.