CRJ-900 Captain reported temporary loss of control and Flight Attendant injury due to unexpected severe turbulence.

Narrative

During cruise flight at FL230 from ZZZ to ZZZ1; we were flying in IMC. Between the fixes ZZZZZ2 and ZZZZZ1 we came out of IMC into VMC and saw at 12 o'clock a towering swirling cell that was not depicted on radar and not alerted to us by ATC. As pilot monitoring; I asked for an immediate left deviation to try and avoid; but it didn't help. We inevitably entered the cell in a left bank and clean configuration. We did not have time to alert the flight attendants. During the severe turbulence; I recall altitude deviations of +/-300' and wide ranges of climbing and descending on the VSI with fluctuations in airspeed. I do not recall how long the turbulence lasted. Once we exited the cell into clear air again; I contacted the flight attendants and was informed one of them had fallen but no other injuries or cabin debris. Later she described to me that she was thrown against the closet corner near the lav and onto the passenger in 1A and the floor. She did not request medical assistance in flight or on the ground; but was audibly shaken and showing signs of an acute stress reaction.After deescalating the flight deck; and receiving an ACARS to fly slow due to another thunderstorm over ZZZ1; I determined that after the turbulence event and the state of my crew; we would be diverting to ZZZ2. My First Officer concurred. At this time; ZZZ2 was less than 100 nm directly in front of us. I had informed ATC of the turbulence and admittedly did not use standard phraseology; but I was also experiencing minor acute stress symptoms. I requested another deviation for more weather buildup in our current direction. In the end; in concurrence with Dispatch; we changed our diversion airport to ZZZ because ZZZ1 had winds above aircraft limitation of 60 knots. Landed in ZZZ with no ssue. Suggestion. Better radar in aircraft. Operable WiFi to connect EFB for secondary radar.