Cowen is expected to make a full recovery, which is expected to take between six and eight months.
The 21-year-old played in all 82 games during his rookie season with the Senators last year, scoring five goals and adding 12 assists. He finished the season with a minus-4 rating.
He also played in all seven games of Ottawa's playoff series against the New York Rangers. He had an assist and four penalty minutes in the post-season.
Cowen played in three American Hockey League games with the Binghamton Senators this season. He had three assists in that span.
Cowen was the Senators' first-round pick — ninth overall — in the 2009 NHL Entry Draft.