Injured an in need of advice please!

[Ratherbelean]

Hi there,

I am a very avid mountain biker and regular gym user. I consider myself to be a fit but slender person (weighing about 155-160lbs, about 4%bf, 6'1, 18yrs old and male)My goal in the gym is to stay lean and strong. Unfortunately in mid July I had a crash while riding and completely tore my ACL as well as partially tore my rotator cuff. Because of this I can no longer ride a bike or preform many gym exercises. I was an instructor and on average rode for 10hrs five-six days a week plus gym workouts when energy permitted. So naturally it's been a massive change to go from 100% to 0%.
It's now mid August and I can visibly see myself atrophy, I've dropped about 5lbs. With surgery lurking around the corner this will only get worse.
I have been trying to eat a med-high protein, medium carb and fat diet to help maintain the muscles I have. Weight gain is not an issue for me as I'm already a controlled eater. Obviously recovering from my injuries is my main priority but I would love to salvage as much of my fitness as possible.
So my question for you guys is simply do you have any advice for me? I feel like there isn't a whole lot I can do other than monitor my eating and wait for recovery. This lack of activity is killing me but any help you may have about exercises/nutrition etc would be very much appreciated!
Thank you in advance!!