The Belgian has two years of his current contract left to run but club chiefs have been keen to secure one of last season's star performers on an extended deal.
His agent does not expect the 24-year-old to stay at the Stadium of Light beyond 2015 but with a series of clubs - thought to include Arsenal, Liverpool and Manchester United - circling, has suggested Sunderland may want to cash in sooner.
"I think it is very unlikely that he will sign a new contract," Nico Vaesen told the Sunderland Echo.
"Simon has obviously come to a point where he wants to step up a level and there is a lot of interest in him.
"He is very ambitious and wants to play in the World Cup, in Brazil, next year.
"It was a good thing that Sunderland stayed up and it is a fantastic club where he has had three great years.
"But it is maybe time to step up a level and play for a club competing for a European spot."
Insisting Sunderland would have the final say, he added: "All teams that are interested in Simon, we direct them to Sunderland.
"He still has a two-year contract there and any interested clubs have to agree a price with Sunderland first. That is the way we conduct our business."