Sergey Sirotkin, who is actually Renault and McLaren's reserve driver, was put on standby to potentially replace Williams' George Russell in Abu Dhabi.
British rookie Russell is not saying what the problem is.
"It's not an illness, it's something different," he is quoted by Auto Hebdo. "Everything will make sense when I can say more.
"I do not feel 100 per cent but I can drive, but the team needed to have a plan B."
France's Auto Hebdo said Mercedes reserve Esteban Ocon was also considered as Williams' potential Abu Dhabi stand-in, but he is too tall.
And 2020 Williams racer Nicholas Latifi does not yet have a super license.
Russell, 21, said he is working on his mysterious problem in Abu Dhabi with Hintsa Performance, the sports coaching company set up by the late F1 doctor Aki Hintsa.
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