Air carrier Captain reported the aircraft encountered moderate to severe turbulence during arrival; resulting in a brief loss of control and a Flight Attendant being injured when they fell.

Narrative

Descending on the ZZZZZ arrival into ZZZ through approximately 9000 ft.; in VMC we encountered moderate turbulence for approximately 2-4 seconds. The aircraft banked to the right and we received a roll authority alert. As quickly as we started to manage the turbulence; the encounter had already ended. Prior to the turbulence event; we were in smooth air from the cruise phase of flight all the way through our descent. I asked the First Officer to call the flight attendants to make sure they were OK. Flight Attendant (FA) XX Stated he fell into a passenger seat and hit his arm on an overhead bin but said he was OK. The rest of the flight went smoothly without incident and after all the passengers deplaned; I debriefed the flight attendants and checked on their well being. During our debrief FA XX stated his arm was sore but thought he was alright. FA XY said she was thrown into her jump seat and that the turbulence was severe. The two remaining Flight Attendants were in the front of the aircraft and did not report being jostled or injured.