The Walkinshaw Andretti United driver popped his shoulder out after this morning's qualifying session while changing out of his driving suit.
The injury was seen to by the team physiotherapist who strapped it to help get Mostert through the Top 10 Shootout and the race.
Mostert would go on to qualify fourth in the Shootout before finishing the race second behind runaway winner Shane van Gisbergen.
Speaking after the race he said the injury was causing him more pain out of the car than in it and joked that it may have helped his results.
“After qualifying I popped my shoulder out getting changed,” said Mostert.
“That wasn’t ideal, but everything in the race felt pretty good. Luckily we had our physio here who was able to tape my shoulder and get on with it.
“But in the racecar it feels better than what it does out of it. Heat and in that position feels pretty good, so probably should do it more often if it comes with these results.”
Mostert explained that it was his second shoulder issue this year, having suffered a similar injury back in March.
"It’s the second shoulder for me, I did one in March," he said. "Left one at the start of the year, right one at the end of the year."
Mostert's second place today followed a third place in Saturday's opener behind van Gisbergen and David Reynolds.
While van Gisbergen's winning margin was muddied by a late safety car, Mostert said he felt closer to the champion-elect than he did on Saturday.
"I’m much happier with the last race," he said. "Shane said yesterday he wanted to put a 20-second gap on everyone so I was pretty motivated in that race to make sure I kept it as little as possible.
"A safety car came at the end and I had a chance to have a crack at him but he was bloody quick this weekend.
"For us to come second after two P4 starting positions, I’m pretty stoked with that."
