Australia is considering drafting a 17th player if the injury to all-rounder Shane Watson is more serious than expected.

In a blow to Watson's chances of playing in the first Ashes Test, a knee injury has ruled him out of Australia's opening tour match against Sussex in Hove.

Results of a scan on Watson's injured knee, which has prevented him from taking part in any of Australia's three training sessions in Hove, will be revealed later on Wednesday.

Team officials, however, have already discussed sending over a replacement for Watson, who was picked on the tour primarily as a spare batsman capable of bowling if needed.

"We might have to do that," said Australian captain Ricky Ponting.

"That's what some of the discussion this afternoon's about, particularly if we happen to have, for an unknown reason, a batter happen to hurt themselves in this game then we could be a little bit short."

Should a reserve be required, Victorian Brad Hodge and South Australian Callum Ferguson would be among the favourites.

Provided scans reveal nothing sinister, Watson could still be fit enough to play in the first Test should any of the top six batsmen break down between now and Cardiff opener on July 8, Ponting said.

"It's not a major thing at the moment and we're hoping it's not anything major that will show up on the scan," Ponting said.

"Hopefully within a couple of days he's right to start back up and work up on his bowling again.

"For him it's obviously a setback. He's disappointed that he wasn't available for selection in this game.

"I don't know if it harms his chances even more for the first Test match but there's an opportunity that's slipped out of his grasp over the next four days."

This injury is the latest in a long line for Watson, who seems perpetually on the comeback trail.

This time, he had been recovering from stress fractures in his back which were diagnosed during last summer's Boxing Day Test in Melbourne.

He returned to the Australian side for the five-match one-day series against Pakistan in the UAE during April and May, playing purely as a batsman.

Watson, who has played only eight Tests in his career, resumed bowling duties earlier this month in Australia's short-lived ICC World Twenty20 campaign.

Meanwhile selectors have decided to rest Mitchell Johnson from the four-day, 12-a-side game in Hove.

He joins back-up wicketkeeper Graham Manou, all-rounder Andrew McDonald and the injured Shane Watson as the four players to sit out this week's match, leaving Brett Lee, Stuart Clark, Peter Siddle, Ben Hilfenhaus and Nathan Hauritz to fight for three places in Australia's Test XI.

Johnson's first match of the tour will be the four-day game against the England Lions in Worcester starting on July 1.

Australia team: Ricky Ponting (c), Phillip Hughes, Simon Katich, Michael Clarke, Michael Hussey, Marcus North, Brad Haddin, Brett Lee, Nathan Hauritz, Peter Siddle, Stuart Clark, Ben Hilfenhaus.