Celtic's miserable week got worse on Sunday as Rangers claimed a second Old Firm win in the space of seven days, this time running out 1-0 winners at Celtic Park through Ryan Naderi's early strike.

It leaves Derek McInnes' side just two points off the top after Rangers' sixth straight win, while Martin O'Neill's Celtic have now lost three on the spin following the League Cup exit at Ibrox and the Europa League defeat to Ferencvaros in midweek.

The result piles further pressure on O'Neill, who is now seriously contemplating his own position at the club.

According to journalist Graham Spiers, the Celtic boss is weighing up his future and plans to properly think things over during the international break, having been left visibly deflated by his side's flat display in the derby defeat. O'Neill is set to head away for a short holiday with his wife over the coming days, with sources suggesting the decision over whether to stay on is playing heavily on his mind.

With Celtic not back in action until the trip to Motherwell on October 11, a 21-day gap, it's been suggested O'Neill could use the extended break to step away entirely, with frustrations having built steadily since the summer.

SFA told they got major Celtic v Rangers incident wrong

Adding to the sense of injustice among the Celtic support, leading referee expert Bobby Madden has since revealed that Rangers should have finished the match with ten men following an incident early in the first half.

Emmanuel Fernandez was shown a yellow card inside the opening five minutes for a foul on Camilo Duran. Not long after, the same defender was involved in a second incident, this time appearing to bring down Benjamin Nygren after the forward had knocked the ball past him, only for referee John Beaton to wave play on rather than stop it.

Madden, speaking to The Daily Record, has warned the SFA that they got this one wrong due to VAR protocols not being able to stop the game for a second yellow card offence.

It raises serious questions over how differently the game, and potentially the wider title race, could have played out. Had Fernandez been sent off inside the opening 16 minutes, Rangers would have faced the vast majority of the match with ten men, handing Celtic a huge advantage to try and force a result.

Instead of sitting two points behind Rangers, Celtic could conceivably be eight points clear at the top of the table had the correct decision been made. Given the growing uncertainty over O'Neill's own future, the miss looks even costlier - a missed red card that, in hindsight, could end up shaping far more than just three points.

Who could replace O'Neill at Celtic?

It also puts fresh focus on who might replace O'Neill if he does walk away. FFC sources understand Kieran McKenna remains the man Celtic would turn to above anyone else, with the Parkhead board known to have been keen on the Northern Irishman even before O'Neill got the job - but McKenna himself has no interest in making the move north of the border.

The 40-year-old is instead holding out for a role at one of England's big six clubs, with Manchester United and Tottenham both firmly on his radar.

Ipswich are also still owed an £8m compensation fee for any move he makes before next July, and Celtic's ownership have shown little appetite to pay that kind of money regardless of how highly they rate him. The hope at Parkhead would be that McKenna is still on the market once that fee drops away, but with the pressure building on both Roberto De Zerbi and Michael Carrick, it's increasingly possible one of those jobs comes up long before Celtic ever get the chance.