Probably stationary, unless you're a long time regular rider and you live in an incredibly super safe area with no traffic, hills, or crappy streets.My Dr's PA recommended bike riding to tighten up the muscles around the knee. I didn't ask if she meant stationary or road bike but I doubt it matters. There have been many postings about how to get the bike set up if you aren't using a bike already.
Stationary because if you fall, then the stakes are high. You should ask what type of bike -recumbent or upright? How long? Interval training or just straight riding? What other exercises and muscle groups are you going to work out? Post surgery, it's best to work with a trainer who understands how best to help those who are in the recovery process.