The former Parkhead star has been wowed by the impact of the Australian manager since arriving in the summer.
Nicholas believes the turnaround achieved under Postecoglou has been most impressive.
"What Ange Postecoglou is building at Celtic is even more exciting than the team Brendan Rodgers had,” he wrote in his Daily Express column.
"I know some people will argue with me because Rodgers won treble after treble and also had that Invincible season.
"There's no denying Rodgers did take Celtic on, but he inherited a team that was used to winning and were already champions.
"His Celtic side was miles ahead of everybody else, including Rangers.
"What is happening with Ange just now is more exciting."