The team says Niskanen is week-to-week. On LTIR, he must miss 10 games and 24 days, which means Niskanen isn't eligible to return until Nov. 7 at Buffalo.
Niskanen left the Capitals' victory at the New Jersey Devils on Friday night. The 30-year-old was averaging almost 26 minutes a game before being injured.
With Niskanen out, Washington recalled 22-year-old defenseman Madison Bowey from the Hershey Bears of the American Hockey League. Bowey could make his NHL debut Saturday night when the Capitals visit the Philadelphia Flyers.
Niskanen's injury puts more pressure on John Carlson, the only other veteran right-handed shooter on the blue line.
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