It's commonplace in sports—you'll most often hear about this in football—to not give your opposition any reason to despise you any more than they already do prior to a big game.
It's called "bulletin board material."
Whether it be Opening Day in baseball or the week leading up to the Super Bowl, it's best to keep any comments to the media about an upcoming opponent as textbook, and as respectful, as possible.
In talking to the media today, the Swedish-native Karlsson had some comments regarding Vezina-candidate goalie—and fellow Frolunda Indians alumni—Henrik Lundqvist.
Before the puck has even dropped on the NHL's second season, Karlsson may have put his team behind the proverbial eight ball simply by opening his mouth. Joking or not, Karlsson's comments show his age and lack of playoff experience.
The rest of the Senators team can't be happy with their young defenseman.