Angry Charlie Nicholas claims Celtic believed rivals Rangers didn't have the financial muscle to unseat them as Scottish football's dominant force.
Neil Lennon and his side are facing a title disaster as their Ten In A Row hopes hang by a thread as they trail Steven Gerrard's side by 22 points.
And Nicholas is certain his former team's disastrous campaign can be traced back to Parkhead powerbrokers not taking Rangers seriously as potential title rivals.
The 58-year-old also believes Gerrard's ability to land defensive transfer bargains in Connor Goldson and Leon Balogun while he believes the Hoops are "brutal" at the back.
Speaking to Paddy Power's From The Horse's Mouth, he said: "Celtic have gone backwards, and that’s because the signings they’ve made have been poor.
"They were lazy about the signings. They have a certain element that the board control the budget. It's been that way for a long time
"But what happened just over a year ago was Rangers lost to Celtic in a League Cup final. That was a warning sign of how Rangers would respond under Steven Gerrard and how Celtic would respond under Neil Lennon.
"The Celtic ownership thought that Rangers don’t really have that much money, so how are they going to make it that much better?
"They allowed £15m-£16m to be spent. People think that's fine and fair, but then it's down to the manager and his staff to do the job to make sure that the signings you're bringing in are better or it's making your squad stronger. That's what Gerrard has done. He's hardly spent any money on defenders, and yet they've only let in six league goals this season.
"They are brutal at the back, Rangers are not a fabulous footballing side. In fact, when they beat Celtic the last two times, their strikers haven’t even played well. Celtic should have won on January 2, but they couldn’t put the ball in the net because Rangers are so strong defensively.
"I hold Celtic guilty, because I thought they got lazy in the type of players that they've acquired.
"They needed strong centre‑backs, they needed full-backs, and they didn’t get them in until late.
"Of course, upfront they've been relying on keeping Odsonne Edouard happy, who's a fabulous player, and then they'll eventually sell him."
Despite his no-holds-barred assessment of his former team - Nicholas insists he's just a Celtic fan at heart
And the former Sky Sports pundit isn't ready to wave the title white flag depsite Rangers' gaudy points lead at the summit of the Premiership.
He added: "The standards have dropped. Rangers have caught them up, and now we're in real danger.
"I've not admitted the title is going to Rangers yet. I'm probably a fool for saying that, but I don’t see any way back for Celtic either.
"It's going to end up a real sad season for them. If you don’t win the 10, then it's a rebuild. That's where I would like the standards to be back to. If they were to go after that type of outlook again, I would be supporting them if that was the case."
