That length of time out will mean the 2009 Test Lion will be unavailable for the key European clashes with the four times continental champions, as well as the PRO12 derby with the Scarlets on Boxing Day.
Meanwhile, Rees is set for a 'lengthy' lay off after suffering a neck injury in the thrilling LV= Cup win over Gloucester last weekend.
The 22-year-old has since had scans on the problem area and may now require an operation to fix the problem.
"We've had Joe scanned and initial discussions, given the nature of his injury, is that he is going to be unavailable for a lengthy period of time and is likely to require surgery," said Ospreys head physio Chris Towers.
"We are looking at the best options to treat and resolve the symptoms and will be in a position to update further once we've completed the consultancy process and can identify all available options.
"Although Adam remains unavailable due to the knee injury he re-aggravated in the Leicester match, we are happy with the progress he is making in his rehab.
"In line with the initial rehab plan, he still requires a further six weeks to get him back to the condition required to reach a position where he will be able to return to playing."
The Ospreys are also without a third specialist tighthead as Aaron Jarvis is currently in camp with the Wales squad ahead of tonight's game with Samoa and the successive Tests against New Zealand and Australia.