Knizner exercised his opt-out clause with the Red Sox on Friday and is now a free agent, Jon Heyman of the New York Post reports.
The veteran catcher inked a minor-league pact with Boston in June and slashed .317/.426/.494 with two homers, 16 RBI, nine runs scored, one stolen base and a 14:13 BB:K across 94 plate appearances with Triple-A Worcester. He'll look to latch on with a team in need of catching help.