The Belgian stopper has been the subject of intense speculation in recent weeks, and looks set for a move to Anfield, but his agent insists the deal has not been completed
Mignolet has played over 100 games for Sunderland since joining in 2010 from Belgian club Sint-Truiden, and Vandermissen believes Liverpool would be a big step up for the Belgian international.
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"If you go from Sunderland to Liverpool it is a big step forward. You have to make decisions at certain times."
Mignolet has played for Belgium 13 times, but is currently behind Chelsea's on-loan Atletico Madrid goalkeeper Thibaut Courtois in the pecking order for the national team.
But Vandermissen doesn't believe the lack of European football will affect Mignolet's chances for Belgium.
"When you play Manchester United, Manchester City, Chelsea and Arsenal every week, this is European football, so what is the problem?" he added.