Unfortunately, I have no clue how much you'll lose -- just wanted to chime in to say:
Don't sweat it! (And, follow your Dr's advise to the T!)
1) I have a buddy who I play soccer with. He tore his achilles and was given a similar timeframe to stay off it -- entirely. He didn't obey -- basically cut it in half or more. He couldn't wait to get back out on the pitch and was out playing soccer again in 4 weeks -- he's regretted it for the last 5 years. The worst part: he can't play soccer at all any more.
2) I've never been "forced" to abstain from exercise for that long a period -- but I've made the choice to do it! (Heh...) Probably happens to me a couple times every year -- I just get sick of working out and completely fall off the wagon. I think my longest stretch has been over 3 months! I certainly lose muscle tone during those periods -- and maybe pick up a little extra flab too -- but it all comes back VERY quickly! Muscle memory amazes me every time...
(If my actual memory only worked that well...!!)
Hope your surgery goes well.
Call it a vacation and enjoy it with NO stress -- just get back on your game when it's safe for you to do so.
Eat this today! (If you don't like that one, come back and click again...)
My Calorie Intake Goal:
Keep the tank full and feel good!
Next physical challenge:
Ride my bicycle from Seattle to SanFran!
Stand at the high point
of every State in the US -- one down (#4 on the list), 49 to go!