Sarkic, who was born in Grimsby, played youth team football for Belgian side Anderlecht before joining Aston Villa.
He had loan spells with Wigan, Stratford Town, Havant & Waterlooville and Livingston before a permanent move to Wolves in 2020.
Loans at Shrewsbury Town, Birmingham City – where he made 23 appearances during the 2021-22 season – and Stoke City followed before his move to Millwall.
Fans have been laying flowers and other tributes at Millwall’s stadium, The Den, where a book of condolence has also been opened.
“Mati was truly proud to represent us and the book will be signed by a lot of people,” added Harris.
“In the coming weeks and months, there will be various tributes – we’ve already started talking behind the scenes about how we can pay our respects and honour Mati’s life but also his time at our football club.”
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