If the person netting and bagging the fish hurt it that badly I would expect internal damage, too, and the fish would die within a day or two.
Rainbow Fish seem more susceptible to Mycobacteriosis than some other fish (though all fish can get it). Do some research. If you have already introduced this fish to your main tank and it has MB then your main tank is now contaminated. If you use a quarantine tank, then you can nuke the tank, and hopefully kill the MB. Isolate this tank until you find out what is really going on.
Nutritional problems as the fish is growing can cause this. No way to fix it now. Not contagious. Since you cannot know if it was nutritional or genetic, do not breed from this fish.
Genetic problems sure can, too. No way to fix it now. Not contagious, just do not breed from this fish.