Sillinger began playing major junior in the Western Hockey League (WHL) with the Pats in 1987-88. After a 43-point rookie campaign, he emerged as a top prospect in the juniors in his second WHL season. With 53 goals and 131 points in 1988-89, Sillinger led the Pats in scoring for the first of three consecutive seasons. The Detroit Red Wings then made Sillinger their first-round pick, selecting him 11th overall in the 1989 NHL Entry Draft. Upon being drafted, Sillinger returned to the Pats for two more seasons, where he posted 129- and 116-point campaigns to receive WHL Second and First All-Star Team honours in 1990 and 1991, respectively.