Martin O'Neill questioned Celtic’s physicality and talked up Hearts’ title chances hours before agreeing to return to Parkhead to take charge, along with Shaun Maloney.
The 73-year-old will be back in the Celtic Park dugout for Wednesday’s visit of Falkirk, 20 years after exiting the club, following Brendan Rodgers’ dramatic and acrimonious departure.
O’Neill’s previous arrival kick-started a period of dominance which has seen Celtic win 18 of the past 25 championships, as well as 23 cup competitions.
But the former Republic of Ireland manager returns with Hearts threatening to disrupt the dynamics of Scottish football after going eight points clear with a 3-1 win over the William Hill Premiership champions on Sunday.
The result sparked an end to a fractious last few months of Rodgers’ second reign, which saw tension over transfer policy made public before principal shareholder Dermot Desmond accused the former Liverpool manager of being "divisive" and "self-serving".
O’Neill had given a frank appraisal of Celtic’s current strength and that of their nearest rivals during an appearance on TalkSPORT earlier on Monday.
"Hearts are on the rise," he said. "I know this time last year Aberdeen won a host of games early on and you knew that was never sustainable. They just couldn’t do it.
"Hearts are a different kettle of fish. They are strong, they have got a bit of backing, and they are physically strong.
"Celtic can still play some beautiful football but you have got to add to that by being a bit physical. When teams are physical against you, you have got to be able to do that.
"Celtic just need to settle down. They have lost their way a little bit, Hearts are riding high, a long way to go."
When asked if this was the season that there could be a "disruptor" in Scottish football, O’Neill said: "Without a doubt, this is it. Celtic are not as physically strong, can actually lose games, whereas before they looked invincible in matches.
"Rangers are no threat whatsoever, although the new manager (Danny Rohl), I think he can maybe do something in time. But they are so far adrift, it’s untrue.
"This is the moment, this is the time now for Hearts. They have gone eight points clear, and that is a decent enough lead, really. And their confidence is growing, and it will grow from that victory as well.
"All things are happening for Hearts, it’s great, whereas just at this minute Celtic are in a wee bit of trouble.
"This is the best time for any side outside the Old Firm to go and win. There is a long way to go but there is momentum gathering at Tynecastle."
The Glasgow stranglehold on the Scottish league has been in place since Aberdeen won a third title in six seasons in 1985, although Hearts led the way for much of the following campaign before losing out to Celtic on goal difference on the final day.
When asked if a Hearts title win would be good for Scottish football, O’Neill said: "For the game itself, absolutely, it would reinvigorate the SP(F)L."
O’Neill returns two decades after leaving to help wife Geraldine through a period of illness, and will join forces with former player Maloney, who was appointed the club’s player pathway manager in June.
