Ronny Deila’s side suffered a 3-1 defeat to Molde in Norway in Group A on Thursday evening.
The hosts took the lead as early as the 11th minute when Ola Kamara steered the ball past goalkeeper Craig Gordon.
Vegard Forren doubled the home team’s advantage in the 18th minute, and although Kris Commons' superb finish halved the deficit in the 55th minute, Mohamed Elyounoussi immediately replied.
Over the course of the 90 minutes, visiting Celtic had 69% of the possession, took 14 shots of which just two were on target, and earned five corners.
Molde had just 31% of the possession, took 11 shots of which five were on target, and earned four corners.
Former Celtic midfielder Petrov was following the match and told BT Sport: “Molde were better organised, they had a plan, they stuck with the plan, and every goal they scored, it was due to a Celtic mistake. It could have been about 10.”
Celtic are at the foot of the Europa League Group A at the moment with just two points from three matches, two points behind second-place Fenerbahce.