Wait, his arm is is not in cast in the final panel. Eh, probably days pass.
Was probably days passed in the final panel. No way Robba-san would put a cast on and put it off after, Kashima will be smart enough to know it's a lie and he's faking it.
Or maybe my "it's an injury type other than broken bones" explanation from the previous page's comments is true; that is, he wears the cast to minimize unnecessary movements in order to speed up healing, and occasionally takes it off to air out the skin as well as test how far he is in the healing process (namely, by seeing how much he can move the injured joint before the pain becomes noticeable).
