Ange Postecoglou accomplished what Brendan Rodgers and Martin O'Neill also did with the domestic clean sweep, which was completed last Saturday at Hampden with a 3-1 victory over Inverness CT in the Scottish Cup Final.
But Nicholas prefers the Ange version above the other two based on spending less than O'Neill, who had a Celtic scoring legend at his disposal, and the fact that Rodgers had no Rangers to go head-to-head with given they were still on their "journey" during his tenure.
He wrote in his Daily Express column: "Ange Postecoglou will leave as Celtic's greatest modern-day manager. Some fans might argue that Martin O'Neill or Brendan Rodgers deserves that label, but not me.
READ MORE: Ange Postecoglou named new Tottenham manager on four-year deal
"O'Neill delivered success at the start of the century with three league titles, three Scottish Cups and one League Cup, but he was given a fortune to spend on players. Chris Sutton, John Hartson, Neil Lennon and Joos Valgaeren cost more than £20million in combined transfer fees, and he had also inherited top talents such as Henrik Larsson and Paul Lambert.
"Rodgers won seven straight trophies from 2016 to 2018, including an unbeaten 'Invincible' debut season and back-to-back trebles. But in reality there was competition as Rangers had just returned to the top flight four years in the lower-league wilderness.
"Callum McGregor and Greg Taylor are the only players he inherited who still get a regular game. Postecoglou delivered five trophies out of six, and his team always played with energy and excitement.
"He has given Parkhead supporters an unbelievable, thrilling two years. I'll be sorry to see him go to Tottenham but he deserves this move to a Premier League giant. He has had a hidden agenda on front.
"The Aussie didn't stand in the way when his players wanted to move on to bigger things - such as Josip Juranovic's move to the Bundesliga - so it shouldn't be a surprise that Postecoglou would do the when the opportunity arose.
"Watching his Celtic teams play gave me a feeling of hope. That was so important against Europe's elite last year because how do you know if you can compete against the best if you don't have a go?
"I would have loved to see him take another crack at the Champions League with Celtic, but it isn't to be. He has left a great legacy and very solid foundations for his successor, who will take over a club and a squad in good health. If things don't work out down south for Postecoglou, I wouldn't hesitate to welcome him back to Parkhead."
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