However Williams' rehabilitation is going well and the Ospreys' physiotherapist Christ Towers is hopeful of an early return.
"Shane is five weeks post-op and is coming along well," he said.
"He is working hard and doing two or three sessions per day, so we are rehabbing him as hard as we can.
"He's making good progress and we aren't ruling out a possible return for the Toulon game (at the end of January), which is in line with our original best-case scenario, although he still has some way to go yet."
Pool 3 has been fiercely contested with just two points separating the top three teams with only two rounds of pool action remaining.
The Ospreys travel to Reading to play London Irish on Sunday January 16 before the final game against Toulon the following weekend at the Liberty Stadium.