Bove and Davis' text on diving medicine lists no direct known problems with dcs risks after knee surgery.
Of course, when DCS only occurs about two to four cases per 10,000 dives, we would have to have an awful lot of people with similar knee repairs dive an awful lot of times to be able to tell anything with certainty. If you could get someone to fund a study of yourself for, say, 100,000 dives, and they would include funding for a dive buddy.. I might be available!!!
I don't know of anything within the known dive medicine literature to contradict what your doc says. There are some theoretical concerns regarding vascularity and bubbling in post-surgical tissues, but we don't have hard data.
Probably just as important to discuss with your doc is the amount of weight you'll be carrying on your back through surf or on a pitching boat deck and be sure he's comfortable with that.
Scuba is a weightless sport..when you're underwater.
On land, the gear weighs a bunch. (It helps us appreciate weightlessness even more)
Good luck.
John