Phillips, 26, had already been ruled out of next month's Autumn Test series against South Africa, New Zealand and Australia after a six-month layoff.
Now the growing concerns among the Wales management that the Ospreys star may be out for even longer have been realised with his comeback delayed by a further month.
An Ospreys statement read: "Mike underwent a minor keyhole procedure last week to help his knee settle down."
It is a major blow for Phillips, who was a key member of Wales' Grand Slam-winning campaign earlier this year.
And the latest set-back throws his involvement in the Ospreys' Heineken Cup double-header with Treviso in December into further doubt as well as Wales' defence of their Six Nations crown.
"The injury hasn't improved as much as we would have liked," said Wales boss Warren Gatland.
"Mike has been working really hard in the gym and made some really good progress, but unfortunately his injury has plateaued out."
Phillips, who claimed the Wales No9 shirt ahead of 2005 Lions Dwayne Peel and Gareth Cooper, had hoped to return to action next month but the Ospreys have insisted they won't be rushing him back.
"He underwent a traumatic operation in the summer and we don't want to rush him back. He has to be 100 per cent," said Ospreys coach Sean Holley.