Sandlak played major junior with the London Knights of the Ontario Hockey League (OHL). Heading into the 1985 NHL Entry Draft he was regarded as one of the top-rated players for the draft. He was selected by the Vancouver Canucks with the fourth overall pick; at 6'4" and 220 lbs he was chosen to address the Canucks' desire for a big scoring forward. He confirmed this status with a dominant performance at the 1986 World Junior Championships, at which he was the captain of the Canadian team and was named the tournament's top forward. Convinced that Sandlak was a better prospect than the slow-developing Cam Neely (who played the same position and style), Canuck management decided that Neely was expendable, and dealt him to the Boston Bruins for Barry Pederson in what would later be labelled by many commentators as one of the worst trades ever made.