Sydney - Key Australian batsman Michael Clarke is struggling to shake off a back injury just days from the first Ashes Test against England, prompting selectors to rush Usman Khawaja into the squad.
Pakistan-born Khawaja, 23, was added as a standby for Clarke, whose long-standing disc problem resurfaced during domestic duties this month and resulted in him missing Monday's three-hour training session.
"Didn't train today, back a little stiff," Clarke wrote on his Twitter feed. "Confident if I keep listening to our physio and doing the right things I will be fit for thurs."
Team physio Alex Kountouris said Clarke's lower back injury was improving.
"He is making good progress in this recovery and further assessment regarding his availability for selection will be made following the next two days of training," he said.
Earlier Kountouris said part of the plan had been for Clarke to not bat Monday, adding: "Hopefully he'll be available for the game.
"I'm reasonably optimistic but until he bats tomorrow (Tuesday) and recovers from that, that's when we'll know how he is. Until he bats tomorrow we can't be absolutely 100 percent sure that he's okay to go," he said.
Kountouris refused to rate the chances of vice-captain Clarke playing the full five-day Test match which begins on Thursday in Brisbane.
"We want him 100 percent fit on Thursday, it doesn't matter what he is today," he said.
National selectors chairman Andrew Hilditch said the doubt over Clarke's fitness meant it was prudent to bring a standby player into the squad.
"Usman has been added as a result of his strong recent domestic form," he said, adding that selectors believed he had the ability to perform well at the international level.
Australia have been hard-hit by injuries in the run-up to the Ashes series, with Clarke, batsman Simon Katich and bowlers Doug Bollinger and Peter Siddle all coming back from a series of strains, knocks and fractures.
Clarke's injury flared during his century for New South Wales against Victoria 10 days ago, forcing him to bat uncomfortably at number eight in the second innings and to be rested for last week's game in Tasmania.
Reports said Clarke also missed training on Sunday and merely walked laps of the field, looking in discomfort as he did so.
Australia had named a 13-man squad for the first Test in Brisbane, instead of the usual 12, as cover for their injury problems. Khawaja was to join the rest of the team later Monday, officials said.
The injuries have compounded a sharp loss of form for Australia, who have won just one game in the last eight and slipped to fifth in the Test rankings before the much-anticipated Ashes series against arch-rivals England.