Willis was twice treated for a head injury in a game with Saracens in March 2013, but was allowed to keep playing.
The 31-year-old was later substituted in the 49th minute of the match.
McHale and Co Solicitors, who are representing Willis, confirmed the existence of the lawsuit but declined to provide any further comment.
A match report on Sale's website from the game with Saracens said: "Sharks were dealt an early blow when scrum-half Cillian Willis appeared to be tackled high and was attended to by the medical team. Thankfully he got to his feet and resumed."
The report later details the former Ireland Under-21 scrum-half being replaced, as he was "struggling after his earlier injury".
Willis, a cousin of former Ireland captain Brian O'Driscoll, made 31 appearances for Sale after joining the Premiership side from Leinster, having previously played for Ulster and Connacht.
Sale have been contacted by the BBC, but are yet to comment.
Former Sale Sharks player Cillian Willis is to sue the club for alleged clinical negligence over a career-ending concussion.