Spurs had made five changes to the team that drew 1-1 with Manchester United.
Dembele admitted: "The most difficult thing is to think every week as you would if you were playing United.
"It is very easy to motivate yourself against them but when you have a team like this [Leeds] it's a bit more difficult, because they have everything to gain.
"You have to give extra energy to motivate yourself and maybe that was a bit of a problem for us but we still feel we can win a trophy this season, because there is a lot of quality in our group.
"It is a bit disappointing that the FA Cup is over but it is great that we have another trophy to get, so we will try to win that one."