Adam Voges has been ruled out of county action for at least six weeks after suffering a torn calf muscle while batting for Middlesex against Somerset at Lord's last week.
Voges, who had been on 86 at the time, sustained the injury to his right leg while jogging a single during Middlesex's first innings, and had to be helped from the field.
After a period of rest, to allow for the swelling to reduce, Voges' calf was scanned on Tuesday when the full extent of the damage was revealed.
He will now return to Australia to recuperate, and intends to return to England in late July in time to play in the NatWest T20 Blast match against Hampshire on August 3.
"Losing Adam is naturally a blow as he contributes significantly to everything we do," Angus Fraser, Middlesex's director of cricket, said, "whether it be with bat in hand, catching at slip or his general presence around the team.
"His absence, however, does give opportunities to other players who are desperate to play in the first XI. In the past we have coped well without an overseas player and I expect us to do so now."
Voges had originally intended to return to Australia after next month's County Championship fixture against Warwickshire at Edgbaston, in order for the county to register Brendon McCullum and Tim Southee as their overseas players for the NatWest T20 Blast.
"Adam has now agreed to come back early at the end of July to be available for these games and all our remaining fixtures," Fraser said. "By then Brendon will have travelled to the Caribbean to play in the CPL. Every indication we have had is that Adam will be fully fit and raring to go by then."