Brendan Rodgers apologised after Celtic lost successive Scottish Premiership games for the first time in 10 years as Hearts earned a shock 2-0 victory at Parkhead on Saturday.
First-half goals from Lawrence Shankland and Stephen Kingsley gave Hearts their first league win at Celtic since 2007, prompting furious Hoops fans to turn their wrath on the club's board of directors.
It was March 2013 when Celtic last lost consecutive league games, against Motherwell and Ross County.
Shankland's free header and Kingsley's 25-yard free-kick saw Hearts end a 14-match losing run at Celtic.
It also secured their first victory in any competition at Parkhead in 25 games, since Michael Stewart's penalty earned a League Cup win in 2009.
The champions' surprise loss, on the back of last weekend's defeat at Kilmarnock, leaves them five points ahead of Rangers at the top of the Premiership, with their Glasgow rivals having two games in hand.
One fan in the main stand sparked wider dissent when he shouted towards the Celtic directors, leading to chants of "sack the board" from around the stadium and more personal songs aimed at chairman Peter Lawwell.
The club's close-season recruitment again came into question with only one of their eight summer signings among the 16 players to feature.
"Our desire and the mentality right from the off was nowhere near the level of a Celtic team. It was so passive it was frightening," Rodgers said.
"It is a real sore one. For the first time I have been here, I would have to apologise to the supporters - because that level is nowhere near the standard of performances required at Celtic.
"It is way off what this club demands. You're sat there in mid-December with 60 000 in and that's how you perform. It is not acceptable."
Elsewhere in the Premiership, Dundee clinched a 1-0 victory at Ross County deep into stoppage-time when Joe Shaughnessy scored with the last touch of the game.
Luke McCowan's free-kick came back off the post and went in off the Dundee captain.
The other two games both finished goalless but only after missed penalties in each.
St Mirren's Mark O'Hara had his spot-kick early in the second half saved by Liam Kelly as Motherwell secured a goalless draw.
Bottom-of-the-table Livingston also wasted the chance to pick up a much-needed win against Kilmarnock after Bruce Anderson missed a late penalty when Stuart Findlay was penalised for handball.