Riendeau would spend the 1986–87 and 1987–88 seasons playing for Montreal's minor league affiliate, the Sherbrooke Canadiens of the AHL. In his first professional season, Riendeau was awarded the Harry "Hap" Holmes Trophy for allowing the fewest goals-against in the AHL. Despite posting winning records in the minors, Riendeau suffered in his only appearance for Montreal during the 1987–88 season, in which he allowed 5 goals in 36 minutes. With Montreal already having excellent goaltending from Patrick Roy and Brian Hayward, Riendeau was traded to St. Louis on 9 August 1988 with Sergio Momesso for Jocelyn Lemieux, Darrell May and St. Louis's 2nd choice (which became Patrice Brisebois) in The 1989 NHL Entry Draft.