Verstappen's future at Red Bull was thrown into doubt last month after his father, Jos, said the team was in danger of being "torn apart" if Horner remained its principal.
It came after allegations against Horner of inappropriate behaviour towards a woman colleague. He denied the claims and an internal investigation cleared him of wrongdoing.
Verstappen has publicly committed his future to Red Bull, but following the Dutchman's dominant victory at the Chinese Grand Prix on Sunday, Mercedes boss Wolff stirred the pot anew.
Wolff told reporters in Shanghai that "everyone is waiting to see what he (Verstappen) is going to do".
"There are so many factors that play a role for a driver joining," said Wolff, who needs to replace seven-time world champion Lewis Hamilton, who is leaving for Ferrari next year.
"Clearly, from the most rational point of view, that's the quickest car in the hands of the quickest driver, but I don't think this is the only reason you stay where you are."
Earlier this season Wolff said that he "would love" to sign the reigning world champion Verstappen if he became available.
His latest comments triggered a prickly response from Horner, who said that Wolff should focus on trying to revitalise Mercedes following a poor start to the season by their high standards.
"Why would you want to leave this team?" Horner said.
"Mercedes are third behind their customers (McLaren and Aston Martin). His time would be better spent perhaps focusing on the team rather than the driver market.
"I can assure you that there is no ambiguity about where Max Verstappen will be next year."
