Kubica was expected to take the Williams F1 seat once again next year after racing for the team in 2017.
However, there is pressure for the British-based team to replace the Pole with Russian Sirotkin.
And Williams will delay announcing their driver line-up for 2018 until late January.
Autosport claim Williams are still working on who will partner Lance Stroll next season.
Kubica was the assumed man to take the seat but that has all changed in recent days.
Both Sirotkin and Kubica tested together in Abu Dhabi last month, with the latter providing the faster times.
However, experience and lap times may not be enough to save Kubica.
For Sirotkin supposedly possesses the weight of Russia behind him and with that a possible £15million sponsorship package.
Express Sport approached Williams to ask about the presence of a financial package backing Sirotkin from SMP Racing, and they would not comment on the issue.
Williams insist the decision over Sirotkin and Kubica will be based on performance alone.
But the money coming from Russia – who run the Sochi Grand Prix – could prove too much for the team to turn down.
Autosport claim Sirotkin’s contract is close to being signed off, with only small details to be addressed.
Sirotkin, 22, has never raced in F1.
And Williams will not confirm who is in their cars for next year until after Christmas.