Spurs lost 3-0 to London rivals West Ham, in the relegation zone ahead of kick-off, on Sunday.
And Dembele believes taking teams for granted will damage their chances of a top-four finish.
“The top teams have to win these smaller games and you can see it’s the difference with the guys who become champions, they win these smaller games,” he said.
“It’s a mental thing. If we want to be there with the top four we need to win these games.”