Warning; the following is my personal opinion only.... I'm not a doctor, or medical professional, but my opinion has been formed through hearing and reading about MUAs and patients who've had one, plus my own common sense.
I think in most cases I've read about or heard about, an MUA seems to be done for the benefit of the doctor, not the patient. Doctors can be impatient at times, and want to see nearly instant results of their work. But, IMHO, that isn't helpful to the patient early on, if ever. Think about a nice, thick hose in your garden. You turn the water on, and fill it while shutting off the sprayer on the end to keep the water in the hose. Then you try to bend it when it's fully inflated and loaded with water. What happens? It doesn't want to bend, it will bend eventually if you put enough pressure on it, but it will leave a permanent mark in the hose at the place you forced it to bend, and some hoses, I'm picturing those with a weave pattern in them, will possibly begin to pop some of those woven threads, and may also leave a kink in that spot. The hose may have bent under a lot of force, but it was harmed in the process. Next, you turn off the water and remove the sprayer end, allowing the water to drain out. The hose now bends easily and without harm.
The implant your doctor put in your knee area bends perfectly well. He took it out of it's packaging and checked it somehow for function before installing it inside your knee area. Nothing happened to the implant between the time he took it out of it's packaging and the time it was implanted; what changed is your tissues, muscles and so on, are throwing a hissy-fit after being assaulted, and are showing that fit by giving you pain and swelling. Swelling after a TKR is caused by fluid building up in your leg (as I understand it-maybe other factors contribute to swelling also-as I stated I'm not a doctor, but fluid is at least part of the reason for swelling) and hanging out for a while before it slowly dissipates. If you try to force that knee to bend while it's in pain and swollen from a build-up of fluid, it's going to reward you with more pain and more swelling. Those tissues that are already stressed by being filled with fluid may also fray and/or break a little, so to speak, but the end result is more assault and injury to an already injured and stressed area. If you are very, very patient, and allow everything to heal, while slowly building up your strength and muscles in the knee again, when the swelling is gone, including the kind you can't see but is inside, your knee will bend nicely. It may not ever bend as fully as your other knee that wasn't replaced, but it will bend nicely again. Forcing it to bend before it's ready can't be helpful, IMO.
Just remember-it's YOUR knee and YOUR recovery; just because your surgeon operated on it doesn't mean he now owns it. You can listen to his opinion and advice, and the opinions and advice of others, as you choose, and in the end only you make the decision on how you want YOUR knee to recover. After you make as informed a decision as possible, those around you need to support you, regardless of what they would do. And that includes this opinion too!
I still have a lot of swelling in my knee, and even my OS isn't sure why, so for now we've both basically chalked it up to my contrary body which always reacts in the opposite way we think it should. But not once has he ever mentioned an MUA, nor have I ever asked him about one-my belief is he probably doesn't believe bending a swollen, fluid-filled knee under force would be helpful. But again, I've never asked him about it so that's just my opinion. I'm sending good healing thoughts your way; please keep us posted!