The Scottish FA say goalkeeper Allan McGregor's recovery from a groin strain "will take weeks rather than days".
McGregor, who recently joined Beskitas, was replaced after 22 minutes of
Scotland's 3-1 win over Australia.
He will return to his club for rehabilitation but is likely to be a major doubt for the start of Scotland's World Cup qualifying campaign.
"A more specific timescale will only be established once he begins treatment at his club," said the SFA.
As he left Easter Road on crutches, the former Rangers stopper told BBC Scotland: "I don't know what happened. I just felt a strange pain when I hit the ground."
The 30-year-old thought he would certainly miss his new club's opening Turkish Super League match, away to Istanbul, on Sunday.
Scotland coach Craig Levein is concerned with the opening Group A match against Serbia, quickly followed by Macedonia, nearly three weeks away.
"Allan landed awkwardly and somehow he has damaged his groin," said Levein.
"It was bad, certainly going by the pain he was in after the game.
"It was a big disappointment."
McGregor was replaced by debutant Matt Gilks and the Blackpool goalkeeper could be first choice against Serbia, with former number one Craig Gordon still battling to recover from injury.
Scotland World Cup qualifiers
- Scotland v Serbia, 8 September 2012
- Scotland v Macedonia, 11 September 2012
- Wales v Scotland, 12 October 2012
- Belgium v Scotland, 16 October 2012
- Scotland v Wales, 22 March 2013
- Serbia v Scotland, 26 March 2013
- Croatia v Scotland, 7 June 2013
- Scotland v Belgium, 6 September 2013
- Macedonia v Scotland, 10 September 2013
- Scotland v Croatia, 15 October 2013
"The good thing is Gilksy got about 75 minutes and pulled off one outstanding save," said Levein.
"I've got David Marshall as well, but I hope Allan is fit because he has established himself as our number one goalkeeper and we would like him fit and available."
Jordan Rhodes scored his first international goal with a 29th-minute header after Mark Bresciano had thumped a superb volley past McGregor to fire the visitors in front.
Jason Davidson headed into his own net after 63 minutes and Ross McCormack rounded off a heartening performance for the Scots by driving home the third 13 minutes later.
Scotland's fine win came at a cost not just to McGregor, with Rhodes, who scored in his first full start, James Morrison, Alan Hutton and Danny Fox all also coming off the pitch early carrying an injury.
"The two full-backs went off injured and Morrison went off after rolling his ankle," said the national coach.
"Jordan's hamstring was tight at half-time, but Allan is by far the worst of the lot.
"We had five players who came off through injury and I don't like sending players back to their clubs with niggles, particularly after a friendly match.
"I don't mind so much if it's a competitive game, so it's disappointing."