Former Celtic and Manchester United midfielder Liam Miller has died at the age of 36.
Petrov, 38 and now retired, returned to football in 2016 having recovered from acute leukaemia, four years after being diagnosed with the disease.
He added: “My thoughts and prayers are with his family and friends. RIP my friend.”
Chris Sutton, who also played with Miller at Celtic, tweeted: “Terribly sad news about Liam Miller. Taken too young. Thoughts go out to his family.”
Hibs said they were “incredibly saddened” by news of Miller’s death, while Sunderland also paid tribute to their former player.
“We are saddened to hear this evening that former Sunderland midfielder Liam Miller has passed away. Our thoughts are with his family and friends at this time,” the club tweeted.
Kevin Kilbane, who was Miller’s team-mate with the Republic of Ireland, tweeted: “Sad hearing the awful news that Liam Miller has passed away this evening, taken far too soon by a terrible disease. Proud to have shared a dressing room with him over the years.”
Kevin Doyle, who also played with Miller for the Republic of Ireland, tweeted: “Extremely sad news. Rest in peace Liam Miller, a gentleman.”
Danny Higginbotham, a former Sunderland team-mate, described Miller as a “great player but more importantly a fantastic person”.
“So sad,” he added. “Devastating news. My thoughts are with his family.”
Leeds said everyone at the club was “deeply saddened”, adding: “Our thoughts are with his family and friends.”
Mark Burchill, who played with Miller at Celtic, tweeted: “Can’t believe my old team-mate Liam Miller has passed away today. Taken far too soon. Horrific news. Thoughts and prayers with his family at this time.”
*culled from BBC