The Hoops have lost their last two Champions League games, while in the previous two Scottish Premiership matches, they have earned victories by a one goal-margin, going down to ten men against Motherwell.
The second defeat to RB Leipzig has left their Champions League knockout qualification hopes teetering on the edge and they will have to hope for results to go their way to pull it off.
Hartson thinks Celtic have failed to get in the flow of things in the last two to four games, with the frontline in particular misfiring.
The Celtic legend believes Kyogo Furuhashi and Georgios Giakoumakis cannot be faulted too much, but their performances in the Champions League have left something to be desired.
Speaking on the Go Radio Football Show, Hartson said: “In the last two or three or four games it’s not flowing.
“It’s not flowing and the strikers are not coming onto the ball and dispatching opportunities for the class and the level that they are at in terms of the player that they are.
“You can’t criticise Kyogo, he’s a wonderful player, Giakoumakis’s numbers are brilliant.
“It’s just simply not happened in the Champions League for them in the four games.”
Celtic are still top of the table in the league and will look to get a win against Shakhtar Donetsk later in the month to salvage their Champions League campaign.
