Dembele told Sky Sports News: "It was a bit embarrassing. 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.
"They have good players and against big teams they always play well. They are a good team. They have some good players."
Tottenham are third in the Premier League with nine games left to play this season.