So, 3 weeks ago, I was walking on the street, and it was really icy (Oh Canada

), I slipped, but I didn't want to fall backwards (there was a little old lady behind me), so I somehow used my left leg to make sure I wasn't falling, and man, did that hurt.
No, it's painful only when I bend my left knee, and because of that, I skipped the gym for the last 2 weeks.
I 've been advised no to do leg exercises (weight lifting), and cardio should be limited to stationary bike and low-intensity elliptical.
I took an appointement with a specialist, but It's gonna be next week.
My question is : can I go to the gym and work only my upper body, while doing low-impact cardio? (I know cardio doesn't stimulate legs as well as weight lifting does, but at this point I'm limited).
And when doing upper-body exercises, should I do them seated?
Also, I know it's better to work all the muscle groups each week, so I don't know if my approach is good.
Thanks for your help