Stokes, who has been on the books of Celtic since 2010, has fallen down the pecking order at the Scottish Premiership giants.
The 27-year-old striker has also publicly hit out at Hoops manager Ronny Deila for not using him frequently this campaign.
The Republic of Ireland international was not pleased with manager Deila’s decision to leave him out of the Celtic squad for last weekend’s Scottish Premiership game against Inverness Caledonian Thistle away from home.
While former Celtic striker Hartson believes that Stokes was wrong to publicly criticise Deila, he has asked why the former Hibernian star has been sparingly used so far this campaign.
“Make no mistake, he publicly humiliated Ronny Deila with that message he sent out,” the former Wales international has written in The Scottish Sun. “But here’s the thing I want to know. Why on earth is Deila not picking him?
“That’s the thing which has been lost in all of the debate surrounding the incident since it happened. Everyone’s had their say on what Stokes did.
“But looking at the Celtic team right now I cannot for the life of me understand why he’s so far down the pecking order. You simply can’t tell me he’s not good enough to get into that team.”
Celtic are at the top of the Scottish Premiership table at the moment with 39 points from 16 matches. The Bhoys are four points clear of second-place Aberdeen, who have played a game more.