Alex Ferguson has paid tribute to "true gentleman" Gerard Houllier following the former Liverpool manager's death at the age of 73.
Houllier managed Liverpool for six years until 2004 and won five major trophies with the club, including a memorable treble in 2000-01, as well as the Community Shield in 2001.
The Frenchman was in direct competition with Manchester United great Ferguson during his time at Anfield, the pair forging a close friendship off the pitch.
Reacting to Monday's confirmation of Houllier's passing, a day after undergoing heart surgery, Ferguson told United's website: "The news has come as a total shock this morning.
"Gerard Houllier was still a young man at the age of 73. Gerard became a really good friend during his time at Liverpool.
"We remained great friends after he left and he was always a great ally to have. He had fantastic football knowledge which he gained during his extensive and varied career.
"When we saw each other at UEFA meetings or other events, we would often enjoy a chat, he was always great company and I will miss him dearly. He was a true gentleman.
We are mourning the passing of our treble-winning manager, Gerard Houllier.— Liverpool FC (@LFC) December 14, 2020
The thoughts of everyone at Liverpool Football Club are with Gerard’s family and many friends.
Rest in peace, Gerard Houllier 1947-2020. pic.twitter.com/isHGXIfe5E
"It is a sad day for the football world and my thoughts are with his family at this desperately sad time."
Houllier also had spells at Le Touquet, Noeux-les-Mines and Lens early in his distinguished coaching career, before guiding Paris Saint-Germain to the Ligue 1 title in 1986.
He then worked as technical director and assistant coach of France under Michel Platini in 1988 before taking over as boss in 1992 but resigned a year later after Les Blues failed to reach the World Cup.
However, it was during his time at Liverpool, initially as co-manager alongside Roy Evans and then as outright manager, where he enjoyed his most successful stint in the dugout.
Evans had been in charge since 1994 but stepped aside a few months after Houllier's arrival, conceding at the time that the arrangement was not working.
"Incredibly sad news hearing of the passing of Gerrard Houllier," Evans posted on Twitter.
"A gentleman I have the greatest respect for and what he achieved at Liverpool football club. Condolences to his family. RIP."
Phil Thompson served as Houllier's assistant following Evans' departure and was among those to pay his respects on Monday.
"I'm absolutely devastated and heartbroken at the sad news of the passing of Gerard," he said. "My mate, my colleague, my boss.
"One of the greatest moments of my life was when we come together in 1998. Just to be in his company was an absolute treat. So loyal, so passionate and extremely fierce.
"So many wonderful times, bringing smiles back to people's faces. 2001 should never be forgotten.
"Since we finished, at the end of every conversation we had, I told him I loved him and would always be grateful for him giving a wonderful partnership. RIP Boss."
Houllier later managed Lyon in Ligue 1 before returning to the Premier League for a season with Aston Villa.