Australia defender Rhys Williams could miss the rest of the year with scans confirming he damaged ankle ligaments playing for Middlesbrough.
The 24-year-old defender collapsed in the second half of Middlesbrough's 2-0 victory over Gillingham in England's League Cup on Wednesday and was substituted.
While it remains unclear whether surgery will be required, Middlesbrough believe Williams will miss up to two months of football.
"He's had scans this morning and he's damaged his ankle ligaments, which was what we suspected," Middlesbrough manager Tony Mowbray told the club's official website.
"He will now see a specialist to decide if he needs an operation or not but it's still going to be a considerable time out."
Williams, who was named captain of Middlesbrough earlier in August, has played every match for his club so far this season.
The versatile defender, who can also play in midfield, played in Australia's last international match against Scotland on August 15 and had been expected to be selected in Holger Osieck's squad for the upcoming internationals versus Lebanon and Jordan before injuring his ankle.
Williams' achievements are even more impressive considering his career has been blighted by injury, with the Western Australian having missed out on the 2010 World Cup with a hip problem.
Mowbray revealed he felt sorry for his captain.
"It's a blow both to Rhys and to the club," Mowbray said.
"We were going to depend on his quality and athleticism this year.
"We were hoping he was going to develop a strong relationship with Jonathan Woodgate."
Williams has played 10 times for the Socceroos after making his debut for Australia in 2009.