The midfielder was dismissed for a lunging tackle on Dejaegere, in the first half of their last-32 clash.
Alli lost possession of the ball, before planting his foot below Dejaegere’s knee. Tottenham played 2-2 in the encounter.
Referee Manuel De Sousa had no choice but to hand him a red card. Tottenham crashed out of the competition 3-2 on aggregate.
UEFA have now handed Alli a punishment of three matches on the sidelines in European football.
Spurs are on a six-game unbeaten run in all competitions, with their last defeat having come in the away leg of their Europa League tie with Gent.