It is believed Stroll will join his father Lawrence at Racing Point Force India from Singapore, after the billionaire fashion mogul rescued that team.
“We don’t know when but Lance will probably leave the team,” Williams boss Claire Williams told Speed Week.
“We wish Lance and Lawrence good luck, but it means nothing other than one driver leaving Williams and another taking his place,” she insisted.
Pole Kubica is Williams’ reserve driver, but it is rumoured that the British team will have to sign a ‘pay driver’ to help fill the financial gap left by Stroll.
Williams said: “If Lance actually leaves, it’s more likely than not that Robert will replace him.”
However, she admitted that financial considerations must also be weighed, particularly as Williams is losing title sponsor Martini as well.
“In the current era of sponsorship, it’s not easy for an independent team to reach its goals without paying drivers,” said Williams.
“We are working very hard on the financial side so that we can sign up drivers with more experience. That’s one of our goals for 2019,” she said.
Williams has also lost a key contract as a battery supplier to Formula E.
“Formula E was one project, but there are many more for us such as with Jaguar and Aston Martin,” said Claire.
As for whether a replacement for Martini has been found, Williams answered: “We are having intensive negotiations, but it is unclear whether there will be one main sponsor or a multi-partner solution.
“What I am confident about is that we will have the funding we need.”