Real Madrid and Paris Saint-Germain were among the clubs to pay tribute to Mino Raiola following the agent's death.
Raiola, whose list of clients included Paul Pogba, Zlatan Ibrahimovic, Erling Haaland and Gianluigi Donnarumma, was confirmed to have died on Saturday at the age of 54.
He had been in a critical condition at a Milan hospital.
Madrid tweeted: "Real Madrid C.F., its president and its board of directors deeply lament the passing of Mino Raiola and wish to express their condolences and affection to his family and loved ones."
PSG, who signed Donnarumma from Milan last year, tweeted: "Mino Raiola, charismatic sports agent, passed away today following an illness. The club, through its president, sends its condolences to his family and loved ones."
Serie A leaders Milan posted "our deepest condolences to the Raiola family and all the friends of Mino for their tragic loss," to their official Twitter account, while Inter also expressed their condolences.