Towards the end of Sunday’s Scottish Premiership defeat by Falkirk, Chris Sutton said it was looking like “one of those days” for Celtic. Afterwards, his remarks about Martin O’Neill’s side were much more pointed.

“Martin’s not going to say it, of course, but the club is in regression, the team is in regression,” Sutton stated on Sky Sports.

“They’re just not a good functioning team at this moment in time. It’s hard to see how Celtic are actually set up and it is a bit off the cuff.

“They’re deficient in every area of the team. This current team are miles off it. Where is the plan, what’s the strategy?”

There was some mitigation the leader leaders surprise defeat on Sunday – their sixth in 10 matches in all competitions.

Goalkeeper Scott Bain excelled in goal for Falkirk against his former club – as O’Neill acknowledged – and Falkirk dug in when they needed to get over the line.

They are still four points clear, but some other stats paint a grim picture for Celtic.

Twenty-four shots to Falkirk’s six; nine on target compared to Falkirk’s two (their two goals); more than double the possession and almost three times as many touches in the opposition box. Celtic had 14 corners, Falkirk had none.

Former Falkirk manager Ian McCall was in accord with Sutton, and former Rangers striker Kris Boyd, who said he “couldn’t see what Celtic were trying to do”.

McCall pointed to age and injury taking its toll on on previously peerless performers. “Carter-Vickers, Tierney, McGregor, Forrest – all of them for different reasons aren’t quite what they were,” he said on Sportsound.

“That happens in football, it’s cruel. Whether it’s injury or overuse in Callum McGregor’s case, I would suggest. Their bodies maybe aren’t capable of the levels they once were.

“It’s not a vintage Celtic.”

Europe aside, Celtic’s season had been rattling along relatively well.

They had bounced back from League Cup and league losses to Rangers with a solid win at Motherwell, a resounding 7-1 victory at home to last season’s runners-up Hearts, and a hard-fought 2-1 win from behind at Hibernian.

Wednesday’s 2-2 draw with Dundee United turned out to be a missed opportunity to extend their lead at the top and they now face the prospect of Rangers and, indeed, Hearts getting closer should they win their games in hand.

Besiktas in the Europa League are next up, a home game Celtic really need to win to stand any chance of progress in that competition after starting with three defeats.

Former Celtic goalkeeper Pat Bonner said of O’Neill on BBC Radio Scotland that it’s been “a brilliant weekend for Rangers”, who won their League Cup semi-final with Aberdeen on Saturday.

“They’re in the cup final and Celtic getting beat allows them, when they play their game, their extra game, to close the gap.

“That then puts huge pressure on to Celtic with the European game coming up now in midweek also. That is going to be tough for them to bounce back from this.”

Of course, O’Neill has seen this all before – fairly recently, in fact. He twice stepped back into the dugout in Celtic’s hour of need last season and ultimately led them to a league and cup double.

Indeed, sitting atop the Premiership after 12 games, he was quick to point out in his spiky post-match interview that Celtic are in a better position than at the same stage last season, when they were four points behind leaders Hearts.

“I don’t know whether [supporters] are angry,” O’Neill told BBC Sport Scotland. “Every single time that Celtic lose a game, people can be angry and frustrated. We come under scrutiny and so we should because this is it, the club is used to winning.

“It’s a long season. This time last year, we were in a worse position.”

O’Neill was also asked about the amount of changes that had been made to his team of late.

“Why would you not change the team? We have an abundance of games,” he replied.

“We’re playing European football. We’re trying to hold the Scottish flag up, so we have to change. It’s hard for the same players to play every single week.”

Alan: Sorry to say, but the plot has been lost. Strange bench selection. Even stranger half time sub. Then we take off our best two midfielders and leave an underperforming captain on the pitch. What is going on??

Imran: I think football has changed too much for O’Neill to be a modern manager. I think he should take a back seat and allow a younger, more ambitious manager to take charge. It would be a shame for his legacy to be tarnished and that’s what looks like will happen as no major changes look imminent.

Sean: Very, very Poor. Falkirk deserved the win.

Kevin: Keep the faith Celtic fans… stay strong.

Edward: Dismal display, Johnstone is not in any way superior to Sinisalo, was surprised at Carter Vickers performance. The unthinkable must now be thought. Is Martin O’Neill able to progress this side? If the answer was Forrest, Tounekti and McCowan the question is NOT how to win a football match.

Liam: Enough is enough. This is rapidly turning into a disaster and we need change: from the board to the manager to the coaching staff to the players. So much for quality over quantity.