Head coach Aitor Karanka has said Middlesbrough did not have the same "desire" as Rotherham as his side lost 1-0 to the Millers on Tuesday night.
Boro dominated the first half with David Nugent and Gaston Ramirez both guilty of missing gilt-edged chances, before Lee Frecklington struck late on.
"We didn't play the same way they did," Karanka told BBC Tees.
"They were fighting for every ball as if they were fighting for their lives and we haven't had the same desire."
Middlesbrough are now four points behind Championship leaders Burnley and just two points ahead of third-placed Brighton, but have a game in hand over both sides.
Despite the loss, Karanka refused to be too downbeat and believes his side can learn from their mistakes.
"We could have won by four or five goals but instead we lost," he added.
"The main thing is that we created chances but we have to learn from this."