New Celtic boss Ange Postecoglou was hoping to get the club’s Scottish Premiership campaign off to a winning start and shrug off a Champions League exit, but saw his team fail at Tynecastle.
They fell behind with just eight minutes on the clock when Gary Mackay-Steven struck for Hearts.
Celtic drew level though in the 54th minute through a superb solo goal from Anthony Ralston.
It seemed to be heading towards a 1-1 draw, but with just 90 seconds left up popped John Souttar to head Hearts to a 2-1 win.
The loss is Celtic’s first on the opening day for 24 years, but Hartson does not want any panic buttons to be pressed and stressed the Bhoys are in transition and sticking together is the name of the game.
“Transition period for Celtic.. it will come together but how long I’d be guessing.. takes work, discipline, organisation and good players with a strong mentality to win for Celtic”, Hartson wrote on Twitter.
“No blame game stick together.”
Postecoglou will now prepare his Celtic side for a Europa League qualifier against Czech side Jablonec on Thursday.