Valtteri Bottas remains a doubt for Sunday's Australian GP with the team confirming they will only make a decision on the morning of the race.
The Finn was sixth fastest in qualifying at Albert Park on Saturday, but he was seen limping back to parc ferme and Williams confirmed later that he was suffering from lower back pain.
He was initially sent to the medical centre to be checked out by the FIA doctors before he was later taken to the Alfred Hospital for precautionary checks.
Williams later tweeted: "Valtteri will rest at his hotel tonight and be checked in the morning by medics before any decision is made."
Williams Head of Performance Engineering Rob Smedley added: "The update will be tomorrow. He's going back to his hotel to rest, so he's not being kept in hospital overnight.
"It's absolutely not for me to say [if he can race], that's for the medical people to say.
"He has to go through all the correct checks this evening and tomorrow.
"He has to get through this initial bit, get all the medical care and treatment and have the assessment tomorrow. Then we will know."
Bottas started to develop the problem during Q2 and then he had an off-track moment in Q3, which certainly didn't helped his cause.
"The car performed well and we were fighting with Ferrari throughout," he explained. "Unfortunately towards the end of Q2 I started to feel a pain in my back. Under the circumstances I'm happy with sixth place, but as Felipe [Massa] proved, this is not a true indication of our pace.
"Given the back complaint I had during the session, the team and I felt it sensible to get me checked out at the local hospital."