Celtic’s chances of at least qualifying for the Europa League ended on Tuesday night when they only managed a 1-1 draw against Shakhtar Donetsk at Parkhead.
The Bhoys are condemned to finish bottom of their Champions League group and are set to crash out of Europe before Christmas.
Celtic’s defending has come under scrutiny in Europe this season and Ange Postecoglou has faced some flak for sticking to his attacking principles even against teams such as Real Madrid.
Hartson believes their performances and experience in the Champions League will be a learning process for the squad.
However, the former striker believes more than the defending, Celtic have paid the price for not taking their chances and scoring the big goals.
Hartson wrote on Twitter: “Thought our passing broke down far too much tonight.
“68 attempts, 3 goals in the 5 games. That’s why we are frustrated.
“This level is brutal if you’re slack.
“Let’s win this league and have another bite at CL next year.”
Celtic created a plethora of chances in the first five group games but only managed to hit the back of the net three times.
