Spurs went into Sunday's game at White Hart Lane with a record of nine wins from 11 matches in all competitions.
In contrast, Sam Allardyce's side were just one place above the relegation zone and struggling for goals in the absence of England international Andy Carroll, having scored just four times in their previous six top-flight outings.
But, playing without a recognised striker, West Ham upset the form book to win the London derby and Dembele is keen for Spurs to learn from the experience.
"You look at the top teams. If you want to be champions, you have to win these 'small games'," he told the Tottenham and Wood Green Journal.
"You can see it's the difference with the guys who become champions - they win these games. If we want to be there with the top four we need to win them too.
"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 focused for these kinds of games.
"It's a mental thing. We have to talk with the team and we need to concentrate together. We have a lot of players and you can change the team very quickly, so everyone needs to know it's a difficult season.
"We looked much better in the first half than the second. In the second half we looked very tired and there was not too much passion.
"You have to win these kinds of games if you want to have a good season, so it's a shame.
"I've played a few times against these kinds of teams. We have to know how to play and we have to know that we should be very careful, because these are good teams as well.
"They have a different kind of style but it's the Premier League - you have to be used to these days so it's a shame we weren't prepared.
"You want to go into the international break with a good feeling, so that's a shame as well. The manager was very disappointed, I think it's normal."