Sky Sports pundit Andy Walker says there is growing resentment among Celtic supporters over the 'dull and uninspiring' football served under manager Ronny Deila.
The Scottish Premiership champions failed to win for the second home game running when losing out to Mark McGhee's Motherwell 2-1 at the weekend.
That defeat followed a 0-0 draw against Kilmarnock at Celtic Park on November 21 and Walker believes supporters are now questioning whether the Norwegian 'is up to it'.
Speaking to Sky Sports Now, the former Celtic and Motherwell striker said: "It's their style of play at the moment. It is dull and uninspiring.
"Celtic at home are normally very good. They had a difficult away game last week against St Johnstone and scored three goals but at home in front of your own supporters I think you really need to send a message that you know what you are doing.
"And I think there is a growing feeling of resentment among the Celtic supporters that Ronny Deila might not be up to it. European results have been embarrassing and there is no evidence he is learning from these European defeats.
"I have to say I still think Celtic will go on and win the title, they might secure another cup or two, but there is no evidence that Ronny Deila has the knowledge to get through the Champions League qualifiers next year and for Celtic to lose out on that again would be a real hammer blow for the supporters."
Celtic travel to Tynecastle to play Hearts on Sunday, live on Sky Sports, and Walker says they will have to be wary of Robbie Neilson's side, and Aberdeen, who have closed to within just one point at the top of the table following a 4-0 victory over Kilmarnock at Rugby Park last weekend.
Walker said: "Sunday will be a tough one for Celtic. It always is at Tynecastle and you have to be aware of what your nearest challengers are doing. Aberdeen have recovered well from that awful period in October where they had a five-point lead at the top and all of a sudden within five weeks it was a seven-point deficit.
European results have been embarrassing and there is no evidence [Deila] is learning from these European defeats.
"They have managed to get that back. They still have great attacking players from middle to front. Jonny Hayes, Niall McGinn and Adam Rooney are all in fine form with Kenny McLean just behind them. So if they can put a run together like they did at the start of the season then they could push Celtic all the way.
"However, I think Celtic's strength, resources and ability to do business in the January transfer window will see them come good in the end."
Meanwhile, Walker is expecting Rangers manager Mark Warburton to be active in next month's transfer window following their 2-1 defeat at Falkirk on Saturday, which allowed Hibs to join them at the top of the Championship table.
"Rangers at home are very good but away they have a lot of problems because teams like Falkirk play two up against them and Rangers struggle to cope with that the way they are playing," he said.
"It's not so long ago they were going to Easter Road with the chance to go 11 points clear at the top. Hibs have now drawn that back, are level and the pair meet at Ibrox on December 28 in which will be a fantastic occasion.
"It will be interesting to see what they do in the January transfer window. I think they need a bit of experience. As I said before, they are OK at home but away they need someone with a bit of experience and someone to calm them down and you can imagine Mark Warburton and Davie Weir have enough contacts in English football to get a player or two in on loan."
Saturday's match at the Falkirk Stadium saw referee Willie Collum draw criticism from Warburton after he awarded two penalties - one for either side - both of which the Englishman said were harsh.
Collum is among the elite referees on duty at the European Championship next year and Walker says the Rangers manager will be in good company in France next summer.
"He won't be the only one who has been critical of Willie Collum," he said. "This is supposedly our top referee. He is going to the European Championship and I'm telling you now, brace yourself for a whole series of penalties and red cards whenever Willie's in charge.
"That was a poor decision [Falkirk's penalty] at the weekend, but it's not entirely Willie's fault. I think he should be helped out there by his linesman telling him the player was outside the box. But Falkirk took advantage of it and it's interesting that Mark Warburton now has to handle the pressure."