A sprained fetlock has ruled second favourite Fiveandahalfstar out of Saturday's Australian Derby, changing the complexion of the race.

Trainer Anthony Cummings said the Victoria Derby winner had injured himself during his win in Saturday's BMW against older horses and would be sidelined for at least three months.

The free running gelding has won his Group One races by leading all the way and was expected to set up a fascinating contest with odds-on favourite It's A Dundeel, a noted swooper.

It's A Dundeel and Fiveandahalfstar have dominated betting on the 2400m classic with Philippi the only other runner under double figures on Monday following his win in Saturday's Tulloch Stakes.

Philippi's win was a confidence booster for Mick Kent heading to the Derby and also lifted the spirits of rival trainer Peter Gelagotis.

The Gelagotis-trained Hvasstan put in the worst performance of his career in the Rosehill Guineas won by It's A Dundeel, beating just one other runner after veering off at the turn.

He came to the Rosehill Guineas on the back of a win over Philippi in the Group Two Alister Clarke Stakes at Moonee Valley.

First thoughts were that Hvasstan just hadn't handled the right handed way of running but Gelagotis said his work since indicated he didn't have a problem.

"We are exposing him on a daily basis and he seems comfortable," Gelagotis said.

"It's hard to justify his performance in the Guineas but he might have been a victim of circumstances.

"We forced him forward. Perhaps we should have let him relax and find his feet.

"Or maybe he didn't handle the occasion.

"I was pleased to see Philippi win on Saturday. It franks my horse's form.

"Nothing went right for him in the Guineas so I'm hoping he turns it around on Saturday."

Hvasstan, who finished fourth in the Victoria Derby in which It's a Dundeel was the unplaced favourite, was fourth again in last month's Australian Guineas.

"He deserves at least a Group One placing and I can't wait to get there on Saturday," Gelagotis said.

The final field for the Derby will be declared on Tuesday.