However Derlago's luck would quickly run out in his first pro season. He suffered a severely broken leg which cost him most of the season. He appeared in only 9 NHL games, scoring a promising 4 goals and 8 points.
"Billy D" got off to a slow second season as well. He only had 11 goals and 26 points in 54 games when Vancouver gave up on him. Fearing he was a bit too small and that his broken leg had caused him to lose a step, the Canucks prematurely gave up on him and another fellow promising young player, Rick Vaive. The pair were dealt to Toronto in exchange for Jerry Butler and Tiger Williams, the key to the deal from a Vancouver perspective.
While Williams and Butler gave the Canucks lots of heart and determination, they didn't provide a whole lot of offense. Vaive of course would go on to be one of the most productive goal scorers in Maple Leaf history, including three consecutive 50 goal seasons. Derlago would often serve as Vaive's center, and while he never duplicated the same offensive flare he showed in junior, he enjoyed some success while in Toronto. Four times he scored more than 30 goals, including a career high 40 in 1983-84. His best offensive season came two years earlier when he scored 34 goals and 84 points.
Derlago would only play one game of the 1985-86 season with Toronto before he was traded to Boston in exchange for Tom Fergus. Bill had trouble adjusting in Beantown and halfway through the season he was dealt to Winnipeg for big Wade Campbell. Derlago couldn't get things untracked with the Jets that year or the next, which he split between Winnipeg, Quebec and the minor leagues. It proved to be Derlago's last season of pro hockey in North America. He did play one more year in Switzerland.
Billy Derlago, one of the greatest junior hockey players in the history of game, scored 189 goals in 555 NHL games. He scored 185 goals in his last two season of junior hockey (124 games)!