The 23-year-old, who only made a return from a serious shoulder injury in late December having been sidelined since April 2002, was forced to leave the field early in the second half of his side's Parker Pen Challenge Cup win over French side Montauban on Saturday.
Bath's head of physical preparation, Chris Mallac, said on Tuesday morning: "Iain saw a specialist in London yesterday and the results of the MRI scan revealed that he definitely has a grade three AC Joint injury. We will be determining over the next 48 hours the best option for Iain's treatment.
"One option is to let the injury heal naturally, which will result in Iain being sidelined for six to eight weeks, but if that doesn't go to plan and he needs an operation after that period, then he has effectively lost one and a half months before surgery.
"The second option is surgery, which would result in Iain being sidelined for three months. The critical issue is determining whether to operate now or whether to proceed without surgery."