The 31-year-old has accused two doctors at Sale of wrongfully handling and incorrectly treating two head injuries he sustained during an LV= Cup tie against Saracens on March 10, 2013.
The scrum-half, who previously played for Leinster, Connacht and Ulster, claims to have been concussed by a high tackle in the first half of the cup tie, when the Sale medical staff deemed him fit to play on.
Willis then suffered a second blow early in the second half and again was allowed to continue before he was eventually replaced after 47 minutes, but he never played again.
Dave Swanton, head of media at Sale, told The Guardian that the club would not comment on an ongoing legal matter.
McHale and Co Solicitors, who are representing Willis, confirmed the existence of the lawsuit to BBC Sport, but declined to provide any further comment.