That is pretty much par for the course losing weight by eating a near non protein diet and only fruit and veges to allow his kidney to heal and minimising the workload on his uninjured one with the expected protein break down and excretion of proteins from broken down muscle tissue during his lay up.He actually did lose 15kg
These proteins are larger particles that can be hard on the kidneys glomerular filtration due to their size.
Muscle loss attributes to the weight loss in total kg simply because its heavier than fat.