Neil Lennon’s men slumped to a 2-0 defeat to Rangers in mid-October, and have looked a shadow of their former selves pretty much ever since.
The Northern Irish manager has come under increased scrutiny in recent weeks as results have consistently underwhelmed, and the pressure on his tenure was only heightened by a 1-1 draw against St. Johnstone on Sunday afternoon.
The result leaves the Hoops 13 points adrift of Rangers at the summit of the Scottish Premiership, and speaking in the aftermath of this weekend’s latest setback, Stewart was quick to pick out the defeat against Steven Gerrard’s side as a key factor in their recent slump.
Speaking on BBC Sportscene, he said: “I remember before the Rangers game that Neil Lennon spoke about how the result wouldn’t determine the league, but it would have a psychological effect. It really has decimated Celtic.
"They went into that game off the back of seven victories in a row. A lot of those results, they weren’t great performances though.
"Since that Rangers game, it has fallen apart. I think a lot of the difficulties you could see when they were winning the games have come to the fore since that defeat.
"The big problem is they can’t break teams down and they can’t defend, and they’re being punished for it. Those two are a toxic combination.
"In the wide areas at the moment they don’t have players that are confident and comfortable with committing players 1v1. They don’t get overloads in those areas either.
"It gets too narrow, and teams know how to play against that, and if you’re confident, you can catch Celtic on the counter-attack.”
Celtic had to rely on an 83rd minute equalier from Mohamed Elyounoussi to salvage a point against the Saints.
Lennon’s side have won just once in the league since losing to Rangers, beating Motherwell 4-1 on November 8th.
The Hoops have drawn the rest of their matches in the Scottish Premiership since then, but have also crashed out of the Europa League, taking just one point in the competition so far.