Antonio Conte watched on in despair as his ten-man side were stunned by the lowest-ranked team in the Europa Conference League, who had lost all four of their previous games - suffering a late 2-1 defeat to the Slovenian minnows.
Spurs went down to Amadej Marosa's stoppage-time goal after Harry Kane had earlier cancelled out Tomi Horvat's shock opener.
And Alli, once seen as the future of English football after breaking through at Spurs and winning the Young Player of the Year award in consecutive seasons, had another evening to forget.
The 25-year-old failed to make his mark in the game and Allen, who made 105 top-flight appearances for the north London club from 1984-1988, has insisted the midfielder must 'get back to basics' or even leave the club in order to rediscover his former self.
Speaking on the talkSPORT Breakfast, the former striker said: "What we're seeing is a shadow of the Dele Alli we know, a shadow of the player that can make runs from midfield, score goals, link up with Harry Kane and be devastating.
"I feel for him. It's the most difficult time in his career. There doesn't seem to be a real desire.
"I don't know how hard on the training ground. Unless you do the basic right, you're going to suffer.
"His ability will come through but that hard work needs to be put in.
"Dele Alli needs to go right back to basics and rebuild his career. He's so talented but it might be that he has to leave Spurs now."
talkSPORT Breakfast co-host and Arsenal legend Ray Parlour echoed the sentiments of Allen, insisting he does not know what Spurs should do with Alli.
"His value is going down and I don't know what they'll do with him," claimed the Gunners great.
"Whether he needs a move I dont know, but it's amazing to see just how much he's gone downhill.
"His stats were unbelievable! They were better than [Frank] Lampard's and people like [Paul] Scholes, it's really baffling."
On Spurs' display in general, talkSPORT's Jamie O'Hara let rip at the team in a scathing rant after the match.
And Allen suggested several members of the team who faced Mura have done their chances of starting for Conte absolutely no favours.
"When a new manager comes in there's always a little uplift in the atmosphere at the club," he added.
"If you've been out of favour or haven't been a regular starter it's a great chance to make your mark in the manager's mind.
"Last night, there were too many of them who have put themselves way, way down in the pecking order.
"He won't trust them in the Premier League games coming up. There's a big cup tie against West Ham coming up in a few weeks, they've put themselves in a difficult position in terms of having any argument in those big games.
"Only two players started from the Leeds game and I don't want to be respectful, but it was Spurs' reserve team.
"It was lacklustre, they were inefficient. There was a lack of aggression. Everything that you want to see as a coach from a group of players trying to impress you was missing.
"It was alarming. Ironically they could now finish second in their group which means it's another two play-off games in February.
"Everything was wrong. The defensive display, the way they closed down, the quality of passing, the number of chances created against a team they should be beating."