Henson recently underwent surgery in Germany to try to cure a long-standing groin problem and while it has been deemed a success, he now faces an eight-week conditioning and rehabilitation programme.
Wales kick-off their autumn series in a sell-out clash against New Zealand on November 5, followed by appointments with Fiji, South Africa and Australia.
Henson would have been a key part of Wales coach Mike Ruddock's plans, but he now joins prop Gethin Jenkins on the sidelines.
Jenkins had a shoulder operation and could be out for up to four months, while Henson's Ospreys colleague Shane Williams has had minor groin surgery and is also battling back to full fitness.
Henson, who has not played since returning home from the Lions tour of New Zealand in July, has been troubled by the groin problem for several months.
But careful injury management saw him through last season's Six Nations campaign, plus the Lions tour, before he travelled to Germany to obtain expert medical advice.
Ospreys manager Derwyn Jones said: "Gavin is making great progress - the operation was a success, and he is now pain-free.
"He hasn't played for a while, so he will be going through an intensive conditioning period before we welcome him back onto the pitch with the Ospreys."
In addition to missing Wales' November schedule, Henson will sit out the Ospreys' opening two European Cup games against French giants Stade Francais and Clermont Auvergne.
Ruddock has midfield options with the likes of Sonny Parker, Tom Shanklin, Mark Taylor and Gareth Thomas, but the loss of Henson is a considerable blow as he looks to build on the Six Nations success.