I definitely wouldn't say "career ending." Career altering? Sure, but not career ending. Especially for a QB who doesn't rely on his running ability. The really bad part to me is the time he's going to lose. He's going to have to work his butt of to be ready for next season, and even then that's a very quick timetable for this type of injury.
Just read a tweet, MRI revealed more than expected. Torn ACL, MCL and other structural damage. I'm no expert but that sounds like a career ending injury. Bengals are my home town team even though I'm not a fan I hate to see this happen.