The Premier League side were beaten 2-1 by the Championship side at Elland Road in the fourth round of the competition.
Dembele, who came on as a second-half substitute during the match, admits that Spurs found it difficult to motivate themselves against lower-league opposition.
"I think that is the most difficult thing in football to try and think like every day you are playing against Manchester United," he said. "To motivate yourself to play good against Manchester United is very easy, you don't have to do anything.
"But against teams like this it is more difficult because you have something to lose and they have everything to win. That is the most difficult thing in football.
"I think you have to give extra energy to motivate yourself and so maybe that was a bit of a problem."
Spurs travel to Norwich City on Wednesday.