The Bhoys lost 3-2 to Inverness Caledonian Thistle. Luck wasn't on their side - but nor were the performances of some of their key players...
Before any criticism is dished out to the losing side on Sunday afternoon, it's important to scrutinise those most at fault: the match officials.
Celtic were denied a stonewall penalty for no legitimate reason whatsoever after Josh Meekings handled Leigh Griffiths's shot on the line and, though the Bhoys had later opportunities to keep their treble hopes alive, the decision was a season changer.
But the Hoops didn't help themselves in terms of their performance against Inverness at Hampden Park, either. Adam Matthews was the first star to truly disappoint for Ronny Deila's men.
The 23-year-old was at fault for the first and third goals his side conceded and endured a game to forget. Injury troubles have clearly affected his game this term.
Another member of the back four also failed to deliver, though, with Emilio Izaguirre not making any sort of impact going forward.
It isn't the first time the Honduran has flattered to deceive for Celtic this season and it was as though he froze on the big stage.
Deila's other defenders weren't much better v Inverness but more was expected of Izaguirre with so much at stake.
A surprise name - as Kris Commons is a Celtic hero. What's more, had the penalty that never was been awarded for the Hoops, the 31-year-old would have likely slotted it home and kept his side's dreams of a first domestic treble since 2001 afloat.
In the event, however, many Celtic fans were left asking if they had just seen Commons' worst ever performance for the club.
The midfielder was ineffectual and supporters wondered whether his reputation preceded him when Deila chose to take James Forrest off and not Commons.