The British Lions flyer picked up the injury during Wales' preparations ahead of the Championship showdown with England on Friday.
The dynamic 22-year-old was taken for scans to discover the extent of the damage with worrying question marks now looming over his involvement.
Wales boss Warren Gatland names his side tomorrow but now faces an anxious wait over one of his most potent attacking threats.
Halfpenny, who suffered injury heartbreak during the 2009 Lions tour of South Africa, missed Wales' disastrous Autumn campaign with an ankle injury.
But he returned to fitness in January and was set to line-up with fellow Lion Shane Williams on the Welsh wings.
Wales are already without fellow tourists Gavin Henson, Tom Shanklin, Adam Jones and Gethin Jenkins.