Williams is known to be a long-term target for the Buccaneers, who are expected to make a move for the shot-stopper during the upcoming Transfer Window.
The Citizen source in the Pirates camp has since disclosed that Mhlongo is likely to be part of a swap deal that will see him join SuperSport, while Williams moves to Bucs.
“I believe the negotiations are at an advanced stage right now," the source said.
"The technical team wants to fix their goalkeeping department and Williams has been identified as a perfect signing," the source added.
"He (Mhlongo) already knows about the situation, but the news has not sat well with him," the source concluded.
Mhlongo, who made 14 league appearances during the 2016/17 PSL season, has fallen down the pecking order at Pirates.
Jackson Mabokgwane and Siyabonga Mpontshane are the two goalkeepers who have been preferred by Bucs coach Kjell Jonevret in recent times.