But Federer's long-time coach Ivan Ljubicic has played down fears the tennis world has seen the last of the 20-times grand slam champion, who turned 40 in August.
Federer has played just 13 matches in 2021 and not one since suffering a straight-sets quarter-final loss to Polish young gun Hubert Hurkacz in July.
The six-time Open champion's latest four-month lay-off comes after he spent a year on the sidelines following his record-setting 15th semi-final appearance at Melbourne Park in 2020.
"Roger Federer is very unlikely to play the Australian Open. He is 40 and doesn't recover as fast as before. But he wants to compete again and won't retire all of a sudden," Ljubicic told Sky Sport TV on Sunday.
"We will be able to see Roger again next year. I don't know when exactly, but he is undergoing rehabilitation. He is recovering slowly, he is not in a hurry."
The veteran superstar was still on crutches when he made a surprise appearance courtside at the Laver Cup in Boston in late September.
"Given that he was still on crutches, I doubt we'd see him here because he needs a longer prep to get ready for a grand slam than what he's going to be able to have," Australian doubles great turned commentator Todd Woodbridge told AAP.
Federer's expected absence, while not a surprise, is a huge blow for Open officials, who had hoped the Big Three of Federer, Rafael Nadal and nine-times champion Novak Djokovic would all be back in Melbourne for at least one last time.
Nadal last week signalled his intention to be in Melbourne after his own injury battles in 2021, while Djokovic's participation remains uncertain.
The world No.1 has yet to commit to another title defence in January after refusing to publicly reveal his vaccination status.
Victoria Premier Daniel Andrews is adamant only fully vaccinated players - and fans - will be permitted entry to Melbourne Park for the season's first major.
Federer contested a record 21 successive Opens from 2000 to 2020 and boasts a remarkable 102-15 win-loss record at the tournament.
Tennis Australia is set to stage its official Open launch later this week at Melbourne Park.
