I didn't think an MUA addressed extension problems at all, only flexion, and Josephine just clarified that. As such, why on earth would your surgeon suggest it? I am in the opposite boat to you with relatively poor flexion, but zero extension. My PT has told me I'm "lucky" because good extension often takes much longer to achieve than good flexion. You and I are only a day apart in terms of surgery, so really, it's early days to expect great extension and for those of us who have it, it just seems to be luck of the draw.
Initially, at 2 weeks post-op, my extension was at -5, and though this sounds odd, what helped it get to zero quickly was walking as one would for a "Field Sobriety Test," but with the longest stride I could manage. PT always has me do it and I do it for about 5 minutes, 3x a day. It's certainly not exhausting or painful and it's also good for balance, so a win on all counts. When I do this, I can really feel those hamstrings stretching in a good way. Along with time, I feel it's also what has helped me begin to walk in a *normal* fashion again as of this weekend; exciting stuff!
So, all this to say, don't despair, stick to your gut instincts, and don't let your OS intimidate you. If you're not up to a confrontation with him, just tell him you need more time to see how things develop naturally.