As one of Scottish football's more controversial and colourful characters, the former Hoops striker had plenty of run-ins with the club during his own playing days.

But McAvennie believes McGeady overstepped the mark when he indulged in a dressing-room row with his manager, which resulted in the winger being fined and suspended from first-team activities for a fortnight.

"I think you have to back the manager," said McAvennie.

"It's an unwritten law - you don't have a go at the manager in the dressing room whether you are right or wrong.

"When I was at Celtic, Billy McNeil fined me quite a few times but not because I answered back.

"He knew he could play me on the Saturday no matter what problems I had. He knew when I crossed that line that I would give 100%.

"So there is something wrong that Gordon Strachan doesn't trust Aiden to do that, because he is a quality player and they will miss him.

"When Aiden McGeady is on form, he is one of the best players in Scotland."

Asked if Strachan could suffer as a result of making a point, McAvennie added: "I don't think so. It might be his making.

"You've got to back the manager. I know Gordon and I don't think he would do it just for spite. Aiden is obviously in the wrong."

Celtic demonstrated their ability to cope without McGeady when they ran out 3-0 winners at Falkirk on Sunday.

But former Rangers star Terry Butcher believes Walter Smith's men will welcome the prospect of the highly-rated player being absent for Saturday's crucial Old Firm derby.

He said: "Celtic have dealt with it well on the pitch at Falkirk, they got on with their job well and it was a professional display.

"They got the three points and the players who came in proved a point that they want to be in the reckoning for the Old Firm game at the weekend.

"They have won the first game and are now looking forward to the second game.

"But, if I was a Rangers player, I would be quite pleased that he's not going to play.

"He's definitely a potential match-winner and has caused Rangers problems in the past."

Some players, including Rangers duo Kyle Lafferty and Maurice Edu and Celtic's Koki Mizuno could make their Old Firm debuts this weekend.

And, although claiming the famous fixture is nowhere near as controversial as the ones he enjoyed as a player, McAvennie insists any debutants will be blown away by the atmosphere.

"I don't think they are as fiery now, I think all that nonsense is finished now," he said.

"There are too many players playing for the money in today's game.

"But, if you don't know what the Old Firm game is all about, you will know when you walk onto that pitch.

"It will never change with the supporters, it's still the biggest game in the world.

"It will frighten a lot of people. I don't know if there will be any debuts and I don't think Mizuno will play.

"But, if he does, it will be like a rocket up his backside."

Meanwhile, McAvennie believes the clash at Ibrox could determine whether the Scottish champions need to spend in the January transfer window.

"I don't think Celtic have spent serious money for a while," he said.

"They haven't bought what I would call a jersey-seller for the last couple of years. I think Chris Sutton was the last big player they spent money on.

"Gordon has done well and you can't argue with his record.

"Whether it's enough for the Old Firm game on Saturday, I don't know."

Butcher and McAvennie were speaking at Setanta's promotion of their live coverage of the Old Firm derby.