The English attacking midfielder has endured a tough time under Jose Mourinho at the club.
Alli has not featured much this season, but grabbed a chance against Wolfsberger with both hands, scoring a spectacular overhead kick to get Spurs first goal in the 4-0 second leg Europa League round of 32 success.
"We started very well, after that we let them get back into the game too much," Dier told BT Sport of the performance.
"Second half we had complete control. I'm delighted for Dele, a fantastic goal.
"Step by step he is getting back to his best. In football everyone has a short memory, maybe they have forgotten his quality. Maybe tonight has reminded people of that."
"We have big ambitions and that stays the same for the Europa League," Dier added.
"The desperation for a trophy has been there for a long time. It has been very frustrating not to get one and we will be doing everything we can to do that."