Goals from Andros Townsend (from a penalty) and Harry Kane put Spurs 2-0 up but Dembele was unhappy with the way the north London side let the Greeks back into the game.
To improve, the 27-year-old says Tottenham need to do two things.
Firstly, Spurs need to be quicker from the very first minute of every half. And another similar characteristic Dembele wants his side to work on is their ability to hold a lead.
"We need to be ready quicker, from the first minute. We did it in the first half and then in the second half we didn’t expect them to come at us," the Belgian is quoted as saying by the Daily Mail.
"We were too relaxed and this cannot happen again. We need to speak about it. Personally, there are some reasons why in the second half we didn’t do so well but we will talk with the team about it.
"A lot of people have their opinion so we will talk internally. We were surprised by them. It can’t happen again because we could have ended up drawing or losing.
"We need to learn that even when we’re 2-0 up it’s not over yet. That’s the lesson."
Dembele will no doubt hope his side get the hint when they face Stoke City this afternoon.