McLean, who died this weekend at the age of 83, shook up Scottish football along with Ferguson in the 1980s as their United and Aberdeen teams challenged the traditional hegemony of Old Firm pair Celtic and Rangers.
Inside three days in May 1983, Ferguson led Aberdeen to their European Cup Winners' Cup triumph and McLean guided United to their one and only league title.
Former Manchester United boss Ferguson said: "My adversaries in England were always Jose Mourinho, Rafa Benitez or Arsene Wenger. But, believe me, my biggest adversary in football was Jim McLean."
Ferguson described McLean, who managed United from 1971 to 1993, as "one of the greatest coaches to come out of Scotland".
"Certainly no-one ever surpassed him," he added. "Jock Stein recognised that and brought him into the Scotland set-up."
Ferguson's relationship with McLean started in 1964 when the pair met on a coaching course at Largs.
"He helped me greatly then and I knew then that he would go on to become an influential and special coach," he said.
"I was only a young man but his football knowledge was obvious to me. He was intelligent and clear-sighted on football."
McLean was then playing at Dundee but he was to make his name as a coach across the road at Tannadice.
When he made the short journey, United were not even the best team on their own street. A decade or so later they were one of the top teams in Europe.
As well as the title, McLean led United to two League Cup triumphs, six Scottish Cup finals, the European Cup semi-finals and a UEFA Cup final.
Ferguson said: "When you think back, it is almost absurd what he achieved. He took a club and changed it completely. He took players and made them better.
"His legacy in football is that anyone who came across him - whether as an opposing manager or as a player - knows he was a fantastic coach."
Ferguson, who will celebrate his 79th birthday on Hogmanay, said of his friend: "Yes, he could be prickly. But deep down he was a good man. He was always ready to help. I learned so much from him."
He added: "I am so sad at this loss and send my deepest condolences to his family. He was a great coach and a great character."
While McLean was known as a strict disciplinarian, former United captain Maurice Malpas hailed the former Scotland assistant manager as a forward-thinking coach.
"He was never one to shout from the rooftops," Malpas told the PA news agency. "But actually, he was miles ahead of his time. In the mid-80s, we had sports scientists, we had dieticians. We were ridiculed because we had sports psychologists - but those kind of things are bog-standard now.
"That was down to wee Jim and his desire to get every ounce from us. He really was a forward-thinker.
"He was the first person I knew to have a satellite dish. It was the size of his back garden but it allowed him to watch games from South America. He really was a football freak in that sense."
McLean was respected throughout Scottish football and beyond.
Frank McGarvey scored the winner against United in the 1985 Scottish Cup final for Celtic before helping St Mirren beat McLean's team at the same stage two years later, but the striker regrets turning down the chance to sign on at Tannadice in between.
"I really wanted to sign but he wanted me to stay in Dundee, that was a problem," McGarvey said. "I had a family settled in Glasgow and so I went back to St Mirren.
"Jim McLean was a brilliant manager, one of the great managers of the 80s and one of the top five managers ever in Scotland. No doubt.
"His United team got a UEFA Cup final and the European Cup semi-final and they beat some top European teams. United are the only team in the world to have a better record against Barcelona than vice versa.
"So I have regrets about not signing, but I went to St Mirren and won a Scottish Cup with them when people said I wouldn't win anything again. But if I had gone to Dundee United we might have won the UEFA Cup.
"It was very sad for me to hear the news. I liked Jim McLean because of what he did for Scottish football."