Valtteri Bottas joins Fernando Alonso as a second surprise name on the sidelines.
Valtteri Bottas is out of the Australian Grand Prix.
The Finn spent the night in the nearby Alfred hospital after suffering back pain during qualifying in Melbourne on Saturday.
He is at Albert Park on Sunday, but was missing from the official driver group photograph on the grid and the subsequent driver parade.
It is believed Bottas, who was seen still walking gingerly, failed the FIA's mandatory car extraction test, which requires a driver to be able to get out of the car and replace his steering wheel within a specified time.
The team confirmed that the injury was an annular tear in his lower back.
"Following a series of tests, the FIA medical delegate has deemed Valtteri Bottas unable to race today," said Williams an hour before the race.
"The team respect's the FIA's decision."
We are now set to have a 17-car field for the first round of 2015.