But Wolff says Mercedes could have some high-profile alternatives, even though Verstappen has re-signed at Red Bull until 2023, and Leclerc at Ferrari through 2024.
"I would be surprised if both of these young drivers have committed to their teams for five years and closed the door to everywhere else," he told Auto Motor und Sport.
"Contracts are often more complex than the media portrays," Wolff added.
He insists he is not worried about potentially needing to replace Hamilton if the six-time world champion does not agree terms for 2021.
"As long as we have a fast car - because this is our first priority - we will always have a situation where we can choose who drives it," Wolff said.
"We have some juniors we rely on who have done a really good job, like George Russell who drove absolutely faultlessly at Williams and Esteban (Ocon) as well.
"So we have a pool of young drivers that we can access," the Austrian added.