Australia batter Will Pucovski is suffering concussion symptoms after being struck on the head during a training session last week, Cricket Victoria said on Thursday.
Pucovski has suffered nine previous concussions and the latest knock puts his Ashes campaign at risk.
The 23-year-old is expected to be among the contenders to open the batting against England, along with David Warner.
"Will is currently managing his recovery in consultation with CV's medical staff," Cricket Victoria said in a statement.
"He looks forward to returning to training as soon as he is able to do so."
The five-Test Ashes is due to begin in Brisbane on December 8.