The Lions flyer suffered a shoulder injury after being flung to the ground in a heavy tackle by South Africa's Francois Steyn in Wales' 29-25 loss on Saturday. And further examination has revealed Williams, a veteran of two Lions tours, suffered a dislocation which popped straight back in.
He is set to go under the surgeon's knife today as doctors attempt to assess the severity of the injury.
"A scan has revealed that Shane did, as was thought, dislocate his shoulder, before it immediately moved back into place," said physiotherapist Mark Davies.
"He will now undergo exploratory surgery in the next 48 hours to assess the full extent of the damage caused by the dislocation, but we know he will be out of action now for a minimum of 10 weeks."
There are also injury concerns hanging over double Lion Gethin Jenkins and 2009 tourist Andy Powell.
Powell played in the defeat to the Springboks despite carrying a groin injury while Jenkins was a late withdrawal. Both players are now ruled out of Wales' encounter with Fiji on Friday and face a race against time to prove their fitness before the All Blacks come to town.
"Andy will be re-assessed at the start of next week, but he won't figure on Friday night," continued Davies.
"Gethin has had an initial scan which failed to show the extent of his calf problem. He will not feature this Friday night and will undergo another scan this week which should give us a clearer idea of his situation."