Williams test driver Susie Wolff will not replace Valtteri Bottas if the Finn is ruled out of the Malaysian Grand Prix, the team have told BBC Sport.
The 25-year-old was declared unfit by a doctor to race in Australia because of a tear in a disc in his lower back.
But deputy team principal Claire Williams said Wolff would not be under consideration - although she refused to confirm the likely deputy.
"Susie Wolff is our test driver not our reserve driver," she said.
"It's not my place to say whether [Bottas] will be in Malaysia. It's a very small tear and it's a case of waiting.
"I don't want to think about what we will do if he's not fit. But clearly we'd have reserve drivers."
Bottas qualified sixth in Melbourne but was not allowed to race, despite complying with the requirement to get out of the car within five seconds.
Although he extricated himself within the specified time, he was in a lot of pain while doing so and the doctors felt he might have been unable to repeat the feat if the car had stopped during the race. Doctors were also concerned about him doing further damage to the injury.
Williams said they "respected" the decision.
Bottas is considered unlikely to miss the Malaysian race on 29 March but if he does it is not yet clear who would replace him.
The team does not have a designated reserve driver, with only test driver Wolff and development driver Alex Lynn on their books in addition to the race drivers.
It would be unlikely to be Lynn, who has just graduated from GP3 and has very limited F1 experience.
Given the team's links with Mercedes, which supplies Williams's engines, it is possible they would choose the world champions' German reserve driver Pascal Wehrlein, who has experience of both the Mercedes and the Force India from pre-season testing.
In 2014 Wolff, married to Mercedes boss Toto Wolff, was the first woman to take part in the race weekend at the British Grand Prix in 22 years.