Unless you are rolling out of bed in the morning directly onto the mat for practice, your knees shouldn't need a lot of additional warm-up.
Normal activity, like walking is sufficient to keep them properly lubricated and ready to go.
Clearly, sideways stress is not indicated for knee joints. But I've had a lot more problems from nikkyo than knee circles in warm-up.
Sorry if I'm being too personal, but I would listen to Dr.'s advice. Carrying around the equivalent of a 200 lb. man on your back all day is going to place way too much stress on the skeletal system.
What I'd like to know is what is up with the movement that is done in some dojos where your legs are straight, you bend over and place your hands on your knees and then flex the knees a bit and scoot backwards? This has always seemed like the most useless thing I've ever done! I can't figure out what is supposed to be accomplished by this. Any thoughts?