The Belgian warns that his side need to motivate themselves for every game, and not just those against top-quality opposition
Spurs were shocked by their London rivals on Sunday, conceding three goals in the second half as they went down 3-0 at White Hart Lane, and the Belgian is calling on Andre Villas-Boas' men to improve their concentration.
"You see from the top teams that, if they want to be champions, you have to win these small games," Dembele told reporters.
"That is what makes the difference and, if we want to be there with the top four, we need to win these games.
"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 kinds of games. It's a mental thing, so we have to talk as a team and make sure we’re all focused.
"We have a lot of players and we can change the team very quickly, so everyone knows it can be a difficult season.
"When we play against sides like West Ham, we have to be very careful. They have a different kind of style but this is the Premier League and you have to be used to these kinds of days, so it is a shame that we weren't prepared.
"In the second half we looked very tired and we didn't have too much passion. You have to win these kinds of games if you want to have a good season, so it's a shame."
Dembele, who is now hoping to help his country seal a 2014 World Cup berth during the international break, joined Spurs from Fulham last summer.