An air carrier Captain reported a brief loss of control due to turbulence causing a stick shaker and Flight Attendant injury.

Narrative

We were descending via the [STAR] and assigned 300kts. The FO (First Officer) was PF (Pilot Flying). At the top of descent I made the descent announcement and then called the FAs (Flight Attendants) and told them the ATIS at LGA had wind shear advisories in effect and that other aircraft had reported some light-mod chop in the descent and that I wanted them to go ahead and take their seats. About 5 minutes later; while descending through FL250 (still above the cloud layer) the aircraft felt as though it hit a strong updraft and both the airspeed AND the low speed awareness tape rapidly increased. The autopilot disconnected and the stick shaker chirped very briefly. The FO pulled the power back and regained control. I set the speed manually for 270kts. He called for the autopilot back on. Seconds later the aircraft began a second updraft which instantly disconnected the autopilot. The airspeed and low speed awareness tape came back up rapidly again but there was not stick shaker activation. As soon as he had the aircraft under control again; I advised ATC of the event.As I was reaching to call the Flight Attendants; the aft Flight Attendant (FA Person B) called up and said the she was knocked to the floor and that the forward FA (FA Person B) was still laying on the ground up front. l asked her if she was able to obtain an assessment of (FA Person B) condition for me. (FA Person B) called up a few seconds later. She said she had fallen and hurt her leg. I advised Dispatch of the situation and; after another conversation with (FA Person B); we elected to have medical waiting at the gate for her. The rest of the flight was relatively smooth and without incident. Upon reaching the gate; (FA Person B) disarmed the doors but I had her sit down in a first class seat as she was barely able to stand. A police officer came on board right away and sat with her to asses her condition while I stood in the forward FA position to disembark the passengers. I wrote the incident up in the AFML (Aircraft Flight Maintenance Log) and called MX (Maintenance).I'm not sure why the FAs were not in their seats. I know for a fact I told them. I even talked to the FO about my plan to sit them down. As for the turbulence event…the it felt like a strong updraft to me. I looked at the PFD (Primary Flight Display) just in time to see the low speed tape shoot up and the shaker chirp. I don't think that the FO did anything that would have caused this to happen but I also cannot stop replaying the event in my head and wondering if there was something I should have done to see it coming. It happened the second time a few seconds later (minus the shaker) and this helps solidify my belief that it was a severe updraft. We were still fairly above the layer where the light-mod turbulence was reported. It was mostly smooth up until that point.