National cricket selector David Boon said Shane Watson would have been a "nice luxury" for the Ashes opener but believed the injured allrounder's withdrawal from the first Test would not derail Australia's gameplan.

A gutted Watson's shocking luck with injuries continued when he was forced to pull out with a hamstring strain.

Watson had wanted to bowl in a bid to prove his fitness for Thursday's series opener, but was only able to run laps of the Gabba with team physiotherapist Alex Kountouris.

He has been replaced by NSW batsman Michael Clarke.

"It's a luxury to have one of your top six to be able to bowl some overs," Boon said of Watson.

"You need a bit of back-up like when Steve Waugh was younger and Mark (Waugh) a bit as well.

"But it's not an absolute necessity, not a permanent back-up - just a nice luxury."

Boon admitted a "subtle re-think" was needed after Watson's withdrawal but believed Australia's first Test gameplan would not be thrown out the window.

"I think the quality of the players will be able to overcome that (Watson's withdrawal)," he said.

"There might be a subtle re-think on how we will approach it.

"But the players we named were the best to start the Ashes off and we believe they can still win it."

Boon said playing Watson at just 70 per cent was too much of a gamble for such an important Test.

"I think it's very important that we have a very strong, fully fit side," he said.

"Obviously to take somebody in who has a little niggle or something that could possibly go wrong into the first Test of such an important series would be a big gamble."

Boon would not bite when asked how Watson's withdrawal would affect the make-up of Australia's bowling line-up.

NSW seamer Stuart Clark, Queensland speedster Mitchell Johnson and South Australian tearaway Shaun Tait are vying for the third paceman spot behind Glenn McGrath and Brett Lee.

"We've got to take a lot of notice of what the captain says, look at the wicket and the weather forecast, whether it is going to swing or not," Boon said.

"We've got a final preparation day on Wednesday - hopefully we will get that combination right."

Australian vice-captain Adam Gilchrist adopted an "if it ain't broke, don't fix it" attitude when asked about the side's first Test bowling attack.

"If we do only go with only four bowlers - three quicks and a spinner - that's been very successful for us for a long time, particularly in Australian conditions," he said.

Meanwhile, Watson was confident of being fit for the second Test at Adelaide starting on December 1.

"Absolutely. The way I've responded gave me a thought I'd get up for Thursday - I've still got eight to 10 days so I am confident I will be fit for that," he said.

Watson's withdrawal added to a sorry history with injury.

He underwent a shoulder operation after falling awkwardly in the field against the West Indies in last summer's Test series.

He also missed the 2003 World Cup and was forced to re-model his bowling action after breaking down with back stress fractures.

"It (luck) has got to turn around and I've got to keep working hard and I'm sure it will one day," Watson said.

"It's not a good feeling, no. This has been something I've been really working towards for the past six months.

"I feel my game's really improved and I've really earned my spot in this Australian team."

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