The midfielder believes the Belgian's equaliser was the turning point in the 2-1 defeat.
However, an equaliser from Guillaume Gillet following a corner gave the Belgians the confidence to get back into the game, and they went on to secure all three points as Stefano Okaka struck the winner.
And Dier says that the leveller knocked Spurs out of their stride.
“We conceded a sloppy first goal and that gave them the confidence to get back into the game,” he told tottenhamhotspur.com.
“Before they scored we were dominating, but then to concede a goal from a set piece is very disappointing. I think that was the turning point because it gave them the confidence to kick on.”
The 21-year-old also says he and the rest of the squad are disappointed as they know they should have done better in the game.
Given how well they started the contest, with Eriksen’s early goal, Spurs could easily have added more to put the game to bed and take a big step towards the latter stages of the competition.
Instead, they missed several big chances to wrap up the game, with Harry Kane again struggling to find the back of the net, and Dier says they must not only take their chances, but concentrate at the other end as well.
“We started so well, in the first 20 minutes we were very good and we could have even killed the game off by then, but we’ve got to take our chances when we get them and concentrate more at the other end,” he added.