Australia full-back Billy Slater will miss Saturday's World Cup semi-final and would also be a major doubt for the final, were his side to get there.
The Melbourne Storm number one aggravated an old knee injury during Saturday's 62-0 quarter-final victory over the USA at Wrexham.
The 30-year-old was helped from the field early in the second-half, having been hurt in a tackle, and he went for a scan on Saturday night.
But an Australian spokesman says that a clear diagnosis could not be obtained because Slater's left knee was too swollen.
And he confirmed that the player will be on the sidelines next weekend and may not recover in time for the final on Saturday week.
A former World player of the year, Slater is a key member of the Australian side, but they do have plenty of candidates to fill in at full-back.
Greg Inglis (South Sydney), Jaryd Hayne (Parramatta) and Darius Boyd (Newcastle) all play at the position for their clubs but line up in the threequarter line for the Kangaroos.
Inglis is probably favourite to deputise for Slater at Wembley next weekend, having been effective when switched to full-back against the Tomahawks on Saturday.