That was after Rangers ended a lengthy winless run against their foes at the end of December with a 1-0 win at Ibrox thanks to a Ryan Jack strike.
Although Steven Gerrard’s men have played just nine times in the league since, they now trail the Hoops by some distance.
They also crashed out of the Scottish Cup to Aberdeen in a quarter-final replay defeat on home soil.
And explaining why Rangers have fallen away, ex-Celtic striker Hartson thinks they allowed the boost of their Old Firm victory to get to their heads.
Rangers beat Celtic for the first time since April 2016 back in December (Image: GETTY)
Steven Gerrard's Rangers were distracted by their win over Celtic claims Hartson (Image: GETTY)
If you look at the results since then, I don’t know if they took their eye off the ball
John Hartson
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He explained: “I think Rangers got carried away.
“If you look at the results since then, I don’t know if they took their eye off the ball.
“Maybe they went away on the winter break and felt they had Celtic’s number.
“You have to ask, where’s it gone?
“They were almost challenging after that win but they have fallen back and gone out the cup.”
John Hartson believes Celtic will win the league by a 15-point margin (Image: GETTY)
Rangers face Celtic this weekend trailing them by 10 points in the Premiership (Image: GETTY)
Steven Gerrard would 'walk' into Rangers team says pundit
Hartson has even tipped Celtic to extend their lead at the top of the table between now and the end of the season.
That is in spite of the Bhoys having gone through the turmoil of losing influential manager Brendan Rodgers to Leicester.
Former boss Neil Lennon has returned to Parkhead until the end of the year as he looks to help steer the club towards a third successive domestic treble.
Hartson added: “I said Celtic would win the league by 15 points - they are on track to do that.”