Celtic have made a poor start to the season and are already six points behind Rangers in the Scottish Premiership standings.
The Glasgow giants have failed to win in their last two league games and are now set to host Bayer Leverkusen in the Europa League on Thursday night.
Hartson stressed that he is worried about the Celtic side because of their defensive frailties, which has been evident in their recent games.
He insisted that Celtic’s style of football means they will win a lot of games but conceded their inability to keep clean sheets means they will struggle to beat the big teams in Scotland and in Europe.
Hartson said on the Go Radio Football Show: “I am worried because we can’t keep clean sheets.
“We went 2-0 up in Seville last week and we conceded four. Some of the goals were just not good enough.
“We could see that at Livingston, we could see at the weekend. I am worried when we go to places like Aberdeen, and I am worried defensively.
“Defensively, we always look like we are going to concede and that’s the problem with Celtic.
“You will win lots of games but you are going to lose the big games.
“You come up against anybody with any quality, any striker of sorts or a decent number 10, will hurt Celtic.
“They are just leaking goals.”
Celtic have scored the most number of goals in the Scottish Premiership so far but their defensive issues have held them back.
