Sir Alex Ferguson has hailed Walter Smith's "immense" contribution to football following his former assistant's death at the age of 73.
Ferguson has led tributes for Smith who led Rangers to 10 top-flight titles, five Scottish Cups and six League Cups as well as to the UEFA Cup final in 2008.
Rangers announced the death of Smith - also a former Scotland and Everton manager - on Tuesday, with chairman Douglas Park saying the "club legend" had been battling illness.
A former Dundee United and Dumbarton defender, Smith was appointed manager at Ibrox in 1991, succeeding Graeme Souness.
He won 13 major trophies in seven years, including seven league titles in succession, before leaving at the end of the 1997-98 season and subsequently joining Everton.
He stayed at Goodison Park for four seasons before a brief stint as assistant manager to Ferguson at Manchester United in 2004, where they won the FA Cup together.
Ferguson said he was "absolutely devastated" at the loss of his "great friend".
"In all that time you were dealing with a man with great moral compass in how he lived his life and the friendship he offered so many people," he said on United's website.
"His contribution to football with Dundee United, Rangers, Scotland, Everton and Man United was immense.
"He was only at United a short time. But he was fantastic. He listened and showed an interest in people and helped everyone he could."
Smith was appointed Scotland manager in December 2004 and was in charge for three years before returning to Rangers, where he won a further eight trophies before retiring in 2011 after completing another domestic double.
Former Celtic player and manager Sir Kenny Dalglish said Smith was "one of the few able to transcend rivalries".
"Though we were on opposing sides on the pitch, he was a real footballing friend off of it," he said.
Souness and former Rangers chairman Sir David Murray said Scottish football had lost "one of its finest".
"Even in the proud history of Glasgow Rangers, Walter Smith stands out as a colossus. He will never be forgotten," they said in a joint statement to PA.
Smith's loss was also felt at Everton.
"He was one of the very best people I was lucky enough to meet in my lifetime in football," Everton chairman Bill Kenwright told the club's website.
"A man of loyalty, integrity and great talent. Strong when he needed to be but with a mischievous sense of humour that could ease even the most tense situations."
Former Everton striker Wayne Rooney, who was coming through the youth ranks at Goodison Park during Smith's time in charge, said he was "devastated" by the news.
The former England captain tweeted: "Great man and a legend of the game. Condolences go to all his family and friends. RIP Walter."
Australian Associated Press