Stiliyan Petrov suggested on the UEFA Europa League Highlights show on BT Sport Europe on Thursday evening that Celtic’s lack of fitness against Molde was a major reason behind their defeat.
The Scottish Premiership giants suffered a 2-1 home defeat to Molde in the Europa League Group A.
"I can assure you that the fitness is not there," said former Celtic midfielder Petrov.
"After 60 minutes, (they were) legless, not sharp enough. When you are not fit, then you start doubting yourself. I don’t think this team is fit."
Mohamed Elyounoussi took advantage of poor defending from the Hoops in the 21st minute to open the scoring for the visiting Norwegian team.
Kris Commons diverted Nir Bitton's strike into the net to level the match just five minutes later, but Daniel Berg Hestad found the target in the 37th minute to secure victory for Molde.
Nir Bitton was sent off for a professional foul in the second half, as Celtic ended the match with 10 men.
Over the course of the 90 minutes at Celtic Park, Ronny Deila's side had 52% of the possession, took 16 shots of which eight were on target, and earned six corners.
Visitors Molde had 48% of the possession, took nine shots of which two were on target, and earned seven corners.
Celtic are now at the foot of Group A with just two points from four matches, three points behind second-place Fenerbahce.