The arrival of seven summer signings had led to a swell of optimism among the Tottenham support, but the bubble burst on Sunday afternoon when West Ham crushed their London rivals 3-0 at White Hart Lane.
On paper, the contest looked a mis-match. Spurs had almost B£100million worth of new talent at their disposal while the only two new signings West Ham could call on were free transfer Razvan Rat and B£5million man Stewart Downing.
Spurs threw away points at home against West Brom, Norwich, Wigan, Stoke and Fulham last season, but they had no trouble beating weaker opposition this term until Sunday.
Dembele believes any more slip-ups like the one against West Ham could prove costly.
"If we want to be there with the top four we need to win these games," the Spurs midfielder said.
"You see the top teams, if they want to be champions, they have to win these small games and you can see it's the difference with the guys who become champions, they win these small games."
With so many talented new players in their team, Spurs will have to psychologically adjust to the fact that they will be favourites in most of the games this season, Dembele feels.
"It's easier to play against the top teams for the motivation but if we want to be a top team we need to be concentrated for these kind of games," the Belgian said.
"It's a mental thing. We have to talk with the team and we need to concentrate each other. It's normal. We have a lot of players and you can change very quickly the team, so everyone needs to know it's a difficult season."
The victory was West Ham's first away win against Spurs in 14 years, and their biggest at the north London club's ground since 1994.
Unlike in their previous two Barclays Premier League home matches against Chelsea and Norwich, Tottenham failed to stamp their authority on the game in the first half.
West Ham made it hard for the hosts to have possession and Spurs' only real threat came from Andros Townsend on the wing.
Andre Villas-Boas' men now lie in seventh position and they have a tricky game at Aston Villa following the international break.
Dembele revealed that the Tottenham manager was unhappy with his team's efforts against the Hammers.
"He was very disappointed," Dembele said.
"I thought we looked much better in the first half than the second.
"In the second we looked very tired and (there was) not too much passion.
"These kind of games you have to win if you want to have a good season so it's a shame."
One crumb of comfort for Villas-Boas is that he expects Etienne Capoue, Aaron Lennon, Danny Rose and Emmanuel Adebayor to have overcome their respective injuries by the time Spurs play Villa on October 20.