Celtic need a "miracle" to secure a 10th consecutive title despite their "outstanding" quadruple treble feat, says former striker John Hartson.
Neil Lennon's side edged Hearts on penalties on Sunday to make history with a 12th successive domestic trophy.
But they trail Scottish Premiership leaders Rangers by 16 points with three games in hand as they resume league duty against Ross County on Wednesday.
"They've got to pick their form up," said Hartson.
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"It's a huge couple of months now for Neil Lennon as Celtic try to claw back the huge deficit on Rangers. That now is the challenge.
"It might amaze a lot of people that Neil has been getting so much flak. One of the reasons the supporters are so aggrieved, and want to protest the way they have, is because this 10 in a row bid, they feel they're not even putting up a challenge against Rangers.
"Celtic will have to go on a remarkable run now and that might include winning at Ibrox on 2 January. Celtic probably need a small miracle now."
Celtic's Hampden glory was a third successive victory after a rocky spell of two wins in 12 intensified the pressure on manager Lennon.
But Hartson, speaking to BBC Scotland's The Nine, feels Celtic's defensive frailties will continue to undermine their pursuit of an unprecedented 10th title in a row.
The champions have kept one clean sheet in 10 games and let slip a two-goal half-time lead against second-tier Hearts on Sunday before prevailing in the shootout.
"I'm not sure I fancy them [for the title] because of the way they defend," added Hartson. "The defence needs work.
"If Celtic win their three games in hand, the gap becomes seven points. Win at Ibrox, it becomes four and they still have to play Rangers twice.
"So it's not beyond Celtic, but I just don't see them keeping clean sheets."