Having overcome Arsenal in the north London derby and trounced Inter Milan in the Europa League, Andre Villas-Boas' side were riding the crest of a wave little over a week ago.
However, a 3-2 defeat at Liverpool was followed by a 4-1 second leg defeat at Inter, where only away goals saved Tottenham's blushes after extra-time.
The lackadaisical performance at the San Siro was the most concerning aspect and leaves Spurs in need of a morale-boosting victory against Fulham on Sunday.
"It was a bit embarrassing," Dembele told Press Association Sport. "We thought it was going to be easier than this.
"We knew they were going to fight but it's a shame that we let them into the game.
"We had to work much harder but we are through and we have to take lessons from the game into the Fulham game.
"The thing is that if you go to a place with a 3-0 lead you take a different mentality. That was wrong.
"It didn't work, what we wanted to do. But we are through and that is okay."
Tottenham's reward is a clash against Swiss champions Basle at the quarter-final stage of a competition Villas-Boas won as Porto manager two years ago.
Spurs are second favourites to win the competition behind Chelsea, who are also rivalling the north Londoners in the league.
Villas-Boas' side sit third with a two-point advantage over the Blues but their Champions League qualification is still far from secure, making tomorrow's fixture against Dembele's former club particularly important.
"We lost against Liverpool, but I thought we played well," the Belgian said. "Against Inter it was a different game to the one against Liverpool.
"This one against Inter we were not confident, but against Liverpool we were. At the weekend we have to be fully concentrated and go for the win.
"They have good players and against big teams they always play well. They are a good teams. They have some good players."
While Dembele can expect a warm reception from Fulham fans, the same cannot be said for Tottenham team-mate Clint Dempsey.
The American's lengthy stay at Craven Cottage ended on a sour note, with the west Londoners' supporters booing him when the teams met earlier in the season.
"Of course you hope that it doesn't happen again because he did a lot for the club as well," Dembele said. "Let's concentrate on the game.
"It was a very good time at Fulham. I enjoyed it a lot.
"They gave me a lot of confidence. They were behind me all the time so I had a very good experience there and I have a very good feeling about the club. "