The Belgium U212 international visited the Stadium of Light last week and his agent, Nico Vaesen, says the Black Cats are extremely close to finalising a deal.
"Simon has made a choice that he wants to sign for Sunderland and it is now up to the clubs to find an agreement as quickly as possible," Vaesen told The Journal.
"He did have options but he has decided to go to Sunderland. It is still to be finalised but I think it is close now, the clubs are not far apart on a fee. I think we're 99% agreed on everything and there are no problems."