Smith will have scans on his medial ligament on Sunday to determine how long he's out of action for.

He left ANZ Stadium in a knee brace after the ACT Brumbies fell to arch-rivals the NSW Waratahs 28-22 on Saturday night.

Just hours earlier Wallabies coach Robbie Deans confirmed the ARU had secured insurance and clearance from Smith's Japanese club Suntory to pick him to take on the Lions.

Instead, Deans - who names a 25-man squad on Sunday - is preparing to throw young duo Michael Hooper and Liam Gill into the biggest domestic series of his Wallabies stint. Smith would have almost certainly been selected to add to his 110 Tests.

He rocketed into calculations when Australia's premier No.7 David Pocock had a knee reconstruction in March.

Ironically, both Pocock and Smith injured their knees against the Waratahs in separate matches.

The Wallabies hopefuls' injury list wasn't pretty reading for Deans after a tough Australian derby.

Tatafu Polota-Nau has a fractured arm. Pat McCabe got through just 30 minutes before injuring his knee. And inside centres Christian Lealiifano (cork) and Rob Horne (cheek) didn't return after half-time.

"I'm not going to say [Smith] is fine, he's got a bang on the knee and there's concern," Brumbies coach Jake White said.

"Doctors don't want to tell you the worst-case scenario, but at this point in time it looks like it might be his medial ligament. But if it's a six-to-eight-week injury it means he won't be in the Lions series. We'll just wait to see how it pans out once he's had a scan."

Deans was at the ground to watch a brutal match.

He would have arrived hoping to see which players stepped up in a selection battle across the field. But he left the stadium scratching his head and fearing he will be without some significant star power for the Lions series.

Prior to his injury, Smith showed how valuable he would have been with the 32-year-old putting on a breakdown masterclass for Hooper.

Polota-Nau was in a battle with Brumbies veteran Stephen Moore for the Wallabies' No.2 jersey.

They matched each other around the field until the Waratahs' hulking rake was forced to exit with his arm in a sling just before half-time.

There's still a month to go before the first Test against the Lions, but Polota-Nau faces a lengthy stint on the sideline and any involvement in the series is unclear.

McCabe's 2013 campaign is fast turning into a massive nightmare.

He broke his neck on the Wallabies' spring tour last year and after three months in a brace was finally given the all clear to return to playing in March. In his comeback game he injured his hamstring and he's since had to bide his time on the bench.

When Lealiifano left the field, McCabe got a chance to show his worth at inside centre for 40 minutes.

But he could only last 32 before another injury put him back on the sideline.

Lealiifano's problem appears less serious and with plenty of time before the start of Wallabies camp in the first week of June, the Brumbies playmaker is set to be rewarded for his strong season.

Deans' No.1 priority will be doing a health check before counting the costs of a brutal derby.