My Husband Is 67, Going To Have A Knee Replacement

My husband is 67, going to have a knee replacement in the fall, has rhumotory arthritis. He's worked very hard (he's a work-a-holic). His legs muscles have been hurting him a great deal since February. He is getting weeker all the time and tires easily. He never complains so I know they hurt when he talks about it. He has lost over 18 pounds since February - without trying. He had an ultrasound done on his legs to see if it was circulation but his circulation was good. Checking on a pinched nerve in his lower back, but, would that cause muscle loss/weight loss? Not sure what to do next.