The Parkhead side currently sit sixth in the cinch Premiership following Sunday's 1-1 draw with Dundee United.
They face Bayer Leverkusen in the Europa League on Thursday before making the trip north to play Aberdeen at Pittodrie.
Nicholas voiced his concerns about Postecoglou in his Daily Express column following the United draw, branding him the “new Ronny Deila”.
"I am not sure Postecoglou has realised how big Celtic really are before he came in,” he said,
"I believe he has missed a pitch. I think Ange thought he would just come in here and his playing style would get him over the line, no problem. But it is not about style, it is about winning.
"Celtic are in meltdown just now. I know Ange is going to fight and scrap against anything but he is standing there without even his own No. 2 beside him.
"He has no protection or help. The manager is not under pressure over his job. I know it is a results business and his team is not playing well.The responsibility for things on the pitch should lie at the manager's door.”
Nicholas added: "This manager is on the edge.
"I think it is a gigantic week for Celtic and if they don't get results Ange won't be sacked but the pressure will grow on the board.
"It is the biggest week in probably the last 10 years or even longer."