Whoever told you that is misinformed. What you were told is a persistent myth that should have died long ago.
There is no magic "window of opportunity" within which you have to gain ROM, or else all is lost.
These articles explain:
Myth busting: the "window of opportunity" in TKR
Myth busting: the "window of opportunity" JBJS
Read this too:
Myth busting: no pain, no gain
Finally, these:
It's never too late to get more ROM!
It's worth the wait for ROM
By the way, no one can do a MUA without your permission, so don't be frightened by that threat.
Your knee is not lazy or out of condition and it does not need to be "trained" to recover.
It is wounded and it needs time and gentle treatment, in order for those wounded tissues to heal. Man, it's been cut open big-time, has had bone sawed out of it, had artificial objects hammered into it and has then been stitched together again. Is it any wonder that it doesn't feel like being very active?
Send your knee to boot camp now and all those wounded tissues will get more inflamed which, as Tim explained above, will do nothing but slow down your recovery.
All that is stopping your knee from bending more now is the internal and external swelling. Concentrate on getting rid of the swelling and you will find that the bend was there all the time.
-
rest,
elevate,
ice and
take your pain meds by the clock
All the exercise you need to do right now is this:
Heel slides and how to do them