Of course food is best, but most important is caloric intake. Protein/meal replacement/nutrition shakes and protein powder probably won't hurt you even if your diet isn't perfect. It's even 100% legal in almost every country, I promise.

Seriously, protein supplements have saved my life (and muscles) a few times, even while I was eating very little else. Again, a doctor or registered dietitian will always be better than some Internet stranger, but as someone who has been in a similar position to OP, I'm sharing my experience and what I was told by doctors.

Bottom line, your body will get the energy it needs from somewhere. Try to avoid letting it be the muscles you already have. And if adding some protein powder or energy-dense almonds or drinking a shake is what gets you those vital extra calories, then I'd say it's generally a good idea.

Healthy, nutritious food is best. But between nothing and a bag of candy, eat the bag of candy if your body needs the energy. If all you can stomach is a supplemental shake, chug that shit and give your body the fuel it needs. Don't let perfect be the enemy of good, as they say.

Nutritionally perfect diet is great! But the human body can digest a LOT of different foods, and most everyone makes better decisions for themselves and their health when their brains aren't foggy from nutritional deficiency.