The 36-year-old is expected to pull away from Michael Schumacher by claiming his eighth world title this year.
And then the Brit could retire from F1 and leave the Silver Arrows needing a new superstar to lead the team.
Verstappen is widely regarded as the quickest man on the grid, alongside Hamilton, and would be aptly placed to make the move.
However, he refused to be drawn on such speculation – although he did not rule out a transfer either.
“I’m very focused on this year and trying to make it a success.
“So early in the season, with only a basically a shakedown, those kind of things I’m not really thinking about at this stage.”
Verstappen is under contract with Red Bull until 2023 but there is a clause in his deal which could let him walk away for free.
And Horner reckons Mercedes will not think twice about snapping the youngster up should he become available.
“I’m sure that should Lewis decide to stop then Max would naturally be the driver at the top of the list,” said Horner.
“But they also have George Russell, they’ve also got other drivers obviously available to them on.”
“I’m not going to go into what that is. It doesn’t relate to the power unit in any way, it’s just a binary performance at a certain measurement in time.
“As with all these things to force a driver that doesn’t want to be there… it’s more about relationships than contracts, and you only pull a contract out of a drawer when you’ve got a problem, in my experience.”