And the Waratahs are preparing to be without prop Sekope Kepu for their clash with the Rebels due to concussion.

Dempsey, who clutched at his leg in agony in the 44th minute of the Waratahs' 16-point defeat to the Brumbies on Saturday, is expected to be out of action for the next six to 12 weeks.

At best, Dempsey could be available for the Queensland Reds game in round 10, but at worst he won't return until after the June Test window.

Dempsey had an MRI scan on Saturday evening but Waratahs coach Daryl Gibson said more would be known on Monday as to exactly how severe the injury is.

"He's got a mid-foot injury so at the moment we're going to run some more tests on Monday and he's going to get a few more scans," Gibson said. "We're trying to determine how stable his foot is and so he's likely to be out between six to 12 weeks at this stage. Until the test, we're not quite sure."

Waratahs No.8 Jed Holloway, who made a solid return from a hamstring injury in his first game of the year, said he was "gutted" to see his good mate go down.

"Hugely disappointing for Jack and I'm pretty gutted because I was looking forward to creating that combination that we had last year and putting some games together with him," Holloway said. "He's a strong guy and I'm sure he'll come back bigger and better."

Gibson said Wells was a likely candidate to replace Dempsey in the back row.

Wells started at No.8 in the Waratahs' first three matches this year but was relegated to the bench to make way for Holloway.

There is also the option of Ned Hanigan, who made his return from injury off the bench on Saturday, making 30 metres from four runs.

Another option is second-rower Dean Mumm. The Wallabies tall timber has experience at No.6 at state and international level and finished off the season there last year for the Waratahs.

"We've got Michael Wells so that could mean a reshuffle with Jed Holloway back," Gibson said. "Michael Wells is quite comfortable at six. There are three good options there."

Holloway added: "We've got reasonable depth. We've been a little bit unfortunate in our back-row stocks losing Brad Wilkin and Demo hopefully for not too long though. I think we're pretty well covered there with Deano and Ned as well."

Meanwhile, a six-day turnaround could rule Kepu out of the Rebels match following a nasty head-knock.

Kepu, in his 100th game for the Waratahs, failed a Head Injury Assessment and subsequently left the field.

He will be monitored this week, although Gibson did not sound overly optimistic about his chances of lining up at AAMI Park on Friday.

"It's a six-day process, so we're right on the borderline of getting him in time for our short [turnaround] match this week," Gibson said. "He's woken up much better, got a good clear head, still a bit of a sore jaw though."