Pep Guardiola's charges moved to within three points of claiming the league title, as goals from Gabriel Jesus, Ilkay Gundogan and Raheem Sterling overpowered Mauricio Pochettino's charges at Wembley Stadium.
"We started very poorly today and not how we envisaged starting. It was a big hill to climb once they'd scored their two goals. When it went 3-1, the game was over," Dier told Sky Sports News.
"Our mentality needed to be better to start the game in the right way. Man City started on the front foot. Against a team like them, it's difficult if you're two goals behind. We reacted well and at the beginning of the second half we played well too. We didn't have it in us to get back into the game again."
Defeat for Spurs also brought an end to a 14-game unbeaten run in the top flight.