Zubrus confirmed the injury on Monday, two days after the loss. NHL teams aren't required to identify injuries during the playoffs.
Zubrus said he's not sure whether he'll require surgery to repair damaged cartilage. He had arthroscopic surgery on the same knee in the summer of 2005.
He said his knee began hurting during the second-round series against the New York Rangers, which Buffalo won in six games. After playing the first three games against Ottawa, Zubrus was limited to only 4 minutes in Game 4. He sat out the game Saturday.
Acquired at the trade deadline in February in a deal with Washington, Zubrus had four goals and four assists in 19 regular-season games with Buffalo. He added eight assists in 15 playoff games, while playing a key checking role.
A 10-year NHL veteran, Zubrus completed the final year of his contract and is eligible to become an unrestricted free agent on July 1.