The former Aberdeen striker, speaking exclusively to Football Insider, insisted the 3-1 loss could be “damaging” for the rest of the Hoops’ season.
Celtic went 2-0 down to the Norwegian champions at Parkhead before Daizen Maeda’s 79th-minute header gave the Bhoys hope.
However, Hugo Vetlesen scored shortly after to restore the visitors’ two-goal lead.
Whelan admitted the manner of the performance and result will have come as a hammer blow to the dressing room.
He suggested the Bhoys could struggle to rebuild their confidence.
Whelan told Football Insider’s Russell Edge: “Celtic, 17 unbeaten, I feel that it’s an embarrassing result. I feel like they’ve let themselves down because that momentum has just been broken.
“They need it so badly right now. This is a crucial stage of the season and that kind of result can be damaging in the changing room and coming into the next fixtures.
“They must have done their homework on teams and how they play. They’re Norwegian champions and, to be honest, they’re on their winter break. They’ve not played for God knows how long.
“That goes to show how bad Celtic must have been when it comes to defending and managing the game.
“It was a massive surprise to me. I felt that it was a foregone conclusion with the result and Celtic would be coming out victorious.
“But then again, when you drop your levels of performance, when you drop that concentration, you don’t respect the opposition, maybe, to an extent and you’re going in there a little bit too cocky, these results can happen.
“They can be damaging behind the scenes for the rest of the season and to the upcoming games. That confidence has been smashed out of you.”
Celtic will look to bounce back from defeat against Dundee at Parkhead on Sunday.
The Bhoys will then travel to Norway for the second leg of their Europa Conference League knockout playoff tie.
This article was originally posted here