The two sides go head to head on Tuesday and Mignolet believes that Fletcher, who was welcomed back into the Scottish set-up following talks with manager Craig Levein, is one of their biggest threats.
"The Scotland game is a big one for us and we want to win it," Mignolet told Sunderland's official website. "Hopefully we can keep Fletch quiet.
"It's important in terms of qualification, as is the Serbia match. We play the Scots at home and if we want to win our group we have to win our home games. Hopefully we can do that."
Belgium are currently second in Group A.