Celtic's week went from bad to worse after a second defeat to Rangers in the space of seven days, and it's left serious questions hanging over Martin O'Neill's future at the club.

Ryan Naderi's second-half strike proved decisive as Derek McInnes' side dominated despite having less possession, restricting Celtic to just one shot on target and leaving O'Neill's men facing serious questions.

Chris Sutton didn't hold back in his assessment of what he saw from Celtic's players, launching a defence of O'Neill and taking aim at the midfield in particular.

According to journalist Graham Spiers, the manager himself is now seriously weighing up his own future. Spiers reports that O'Neill will use the coming days of the international break to properly consider whether he wants to continue as Celtic manager, having been visibly pained by his side's toothless showing in the 1-0 defeat.

O'Neill is expected to take a brief holiday abroad with his wife during the break, with the question of whether to stay on reportedly weighing heavily on his mind throughout.

With a 21-day gap now until Celtic's next fixture away to Motherwell on October 11, Spiers suggests it wouldn't be a shock if O'Neill used the break to step away from the role altogether, pointing to a number of underlying issues that have built up since the start of the season, including frustration around the failure to replace Arne Engels in the transfer market.

Should O'Neill's Celtic career come to an end, it would thrust the club's succession plan back into focus - and that's where the news gets worse for supporters.

Football FanCast understands that while Kieran McKenna remains Celtic's top managerial target, with the Parkhead hierarchy having held strong interest in the Northern Irishman before O'Neill's own appointment, the former Ipswich manager is not interested in a move to Glasgow.

McKenna is understood to have his sights set on a job at one of England's big six clubs, with Manchester United and Tottenham both firmly in his thinking. There is also the issue of the £8m compensation fee owed to Ipswich for any move he makes before next July, with Celtic's ownership reluctant to spend big despite being big fans of the 40 year-old.

The hope for Celtic would be that McKenna is still available once that compensation window passes - but given the growing pressure on both Roberto De Zerbi and Michael Carrick, it looks increasingly likely one of those two vacancies could open up long before then.

For now, O'Neill's focus remains on the international break. "The non-internationals will get most of this week off," he said. "Then it's a matter of coming back, regrouping really."

Celtic face a tough run on the other side of it, with away trips to Motherwell and Benfica to come before a home game against Hearts.