ST. LOUIS, Mo. — Avalanche winger Mikko Rantanen left Monday’s game against the St. Louis Blues with what could be a serious leg injury. Rantanen, who signed a six-year contract late last month that pays him $12 million this season and next, appeared to toe-pick his left skate into the ice while coming out of the defensive zone early on his first shift second period at Enterprise Center. A team spokesman said he would not return.