Muscle loss question

I recently tore my rotator cuff and have to side line my strength training plan while healing and doing PT. I am now limited to walking on the treadmill...snooze. I had toned up and gained (visually) some pretty sweet muscle over the last few months but now I'm worried I am going to lose it all. Am I? How fast will I lose it? I still have about 30 lbs to lose but I don't want it all to be my muscle. Any thoughts?