The openside suffered the issue during the West Countrymen’s victory over Leicester Tigers last month, hobbling off just after half-time.
Underhill impressed for the Red Rose in November and was particularly outstanding against New Zealand as they succumbed by one point to the defending world champions.
The 22-year-old will not return until mid-April, meaning that the back-row will miss the whole of the competition which runs from February 1 to March 16.
He would have expected to play a big role in the upcoming tournament but it now leaves Sale Sharks‘ Tom Curry as the man in command of the number seven shirt.
[playbuzz-item item=”0ba39b2a-920a-4b3a-bfb9-5b890c278c3a” info=”false” shares=”false” wp-pb-id=”898183″]