MIAMI — Ryan Dempster has not pitched in the major leagues in more than three seasons, and he is not making a comeback. At 39, after nearly 3,000 professional innings, he knows he could not withstand the rigors of a season.

But his country wanted him for the World Baseball Classic, so Dempster, a native of Sechelt, British Columbia, got ready. At first, he could barely throw 80 miles an hour. He built up to 90 m.p.h. or so, and Canada’s manager, Ernie Whitt, named him the starter for the Pool C opener at Marlins Park on Thursday.

“Then I saw the lineup,” Dempster said, “and I was like, ‘Well, maybe not them.’”

Dempster was joking, he said, but maybe only a little. “Them” is the Dominican Republic sluggers, who bruised him for seven hits and four runs in two innings, in a 9-2 romp past Canada to start their team’s title defense. In 2013, the Dominicans won all eight games for their first W.B.C. championship, after Japan’s victories in 2006 and 2009.

Tony Pena, the Dominican manager and longtime Yankees coach, conceded his roster looked strong. But he implored his collection of All-Stars to think otherwise.