The 34-year-old wing wizard damaged cruciate knee ligaments in last weekend's 19-13 RBS 6 Nations win by Wales over Ireland at the Millennium Stadium and will be out for up to eight weeks. The Welsh victory proved pretty costly for the Ospreys, who also lost tight-head prop Craig Mitchell for 16 weeks with a dislocated shoulder.
Unlike Michaell, Williams won't require surgery, but eight weeks will take him up to the Play-Off semi-finals on the weekend of 14/15 May. The Magners League Grand Final is on Saturday, 28 May.
"Shane will be out of action for a period of six to eight weeks after sustaining damage to the posterior cruciate ligament in his left knee during the match at the weekend," said Wales' medical manager Prav Mathema.
"Thankfully there is no need for surgical intervention and he will now undergo a period of rehabilitation which will see him return to full fitness.
Wales coach Warren Gatland has already added Scarlets wing George North to his squad in advance of the trip to Stade de France and there are no further call-ups expected.
The Wales side to face France will now be named on Thursday, 17 March after Gatland took the decision to allow extra training time for players throughout his squad to make their bids for selection.
It is the second major injury the veteran wing has suffered this season. He only returned to fitness in January after dislocating his shoulder in the autumn international clash with South Africa.
During his Magners League career Williams has racked up 71 appearances for the Ospreys scoring 138 points.
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