Liverpool keeper Mignolet, Chelsea's Courtois shocked by Gillet ban
Gillet has been banned for three years and seven months after an investigation into two defeats for Italian side Bari in 2008 and 2009.
Gillet was a mentor to both Mignolet and Courtois.
"Both owe Gillet a lot, and they are stunned by what has happened to him," a source close to the Belgian FA to the Daily Mail.
"He is vastly experienced and was always on hand to help them, with advice or a word of encouragement. He was very much their mentor. They set great store by his guidance, and it is very sad that such a distinguished career should end in these circumstances."