I disagree with what others are saying (note, I am not a dietician or a doctor, just a former athlete and not an elite one), in that I believe you will lose very little muscle mass per se in 1.5 months UNLESS YOU STARVE YOURSELF. Then your body will consume your muscle mass. I agree with the advice to 'eat clean', although eating like you are training is incorrect, you don't need quite that many calories and you'll just bulk up on fat.
What happens is that you will be deconditioned. That's why it comes back so 'quick'. Because you don't have to regrow, just retrain and recondition. When you come back to lifting your muscles will not be storing enough glucose to be able to lift that much, but it only takes a couple of sessions for your muscles to be primed with glucose again. Also, your heart muscle is involved in lifting so make sure it's still in good shape while you're down.
The best thing you can do for yourself is to stay active every day and DON'T tear up your upper body. If that means having someone help you, so be it. I see it as training to deal with clients in my day job who are handicapped all the time, not just for a few weeks.