The Cardiff Blues forward faces a minimum two-month absence while he recovers from an operation to repair ligaments.
It is another blow for Wales' Grand Slam-winning coach Mike Ruddock, whose star backs Gavin Henson and Shane Williams are currently engaged in fitness fights following groin surgery.
Henson and Williams both expect to be fit in time for New Zealand's Millennium Stadium visit on November 5, with Wales then tackling Fiji, South Africa and Australia on successive weekends.
Jenkins, whose try helped Wales clinch the Six Nations Grand Slam last season with victory over Ireland in Cardiff, shone on the Lions tour of New Zealand despite a 3-0 Test series whitewash.
"It is bad news," said Wales team manager Alan Phillips. "But we just have to get on and deal with it."
Wales physiotherapist Mark Davies added: "The operation went well, but the minimum he will be out is two months, so he is definitely out of the autumn internationals."