Air carrier First Officer reported a Flight Attendant suffered minor injuries when they encountered moderate turbulence at FL370 over the Pacific Ocean.

Narrative

I was pilot monitoring at the time of incident ZZZZ ZZZ. We were routed south towards Saipan for apu inop; putting us in line for some weather enroute. Aware of this and approaching our second sigmet; we asked and received 30L/R weather deviation from KZAK. It was limited moonlight visual conditions; night time over the pacific and we were mostly navigating around buildups visually; as well as radar. The First Officer's (FO's) radar range at this time was set to 40miles; and I was switching between 80 and 160 miles selected for longer range SA. During execution of the offset 15L and 20° to intercept; we entered clouds not showing radar returns on 'All'; on our intended course. As we started our right turn to intercept our course 15L in IMC; we maintained situational awareness of the two cells we were avoiding visually saw prior to entering IMC: one left side; one right side. The right cell appeared as a red dot; surrounded by a significant band of yellow surrounding; then scattered yet consistent green surrounding the yellow. The cell on our left was a larger band of yellow than the right cell and had more green surrounding it. The gap between these two cells where we intended to go; was only painting occasional green dots on all. Since we were intending to turn right to join our course; the right cell was of more concern to us. After clearing the cell on the right according to radar; we started turning right to join parallel to our original course as programmed into the Multi-purpose Control Display Unit (MCDU). We popped out of the smooth stratus layer between the two cells. This is when we saw the incident cloud dead ahead of us. We were on lateral deviation; but hard altitude FL370.As the pilot monitoring (PM); I mentioned we should pull heading mode and turn to go around the cell immediately. At the same time; I reached for the handset thinking to do the PA for cabin crew take your jumpseats. While picking up the handset; I instinctively selected Interphone+1 from muscle memory (from previous Bathroom break requests) but recognized my mistake and reset the phone. I then picked it up again; pressed PA+ALL and before I could hit the PTT button; we had entered the cloud. Timewise; this discussion and pa attempt occurred in 15 seconds or less.To our calculations at around 500 kt ground speed; we had less than 2 miles of clear air between exiting IMC; and the developing cell in front of us. We entered the cloud between XA:30 and XA:45.The FO was flying pilot; we were on Autopilot 2 (AP2); seatbelts sign on; and within the cloud we entered the rain. AP2 did its best but disconnected; and the aircraft climbed to an altitude of 37600 feet. To us up front; we thought it felt moderate and as precaution checked in with our effective first flight attendant (FA) (the established first was on break at that time). We asked if everyone was ok; and initially the response was yes; just surprised. I later received a call from the aft with the news no one wants to hear: one of our flight attendants was up in the aft galley and fell during the turbulence; and was injured.She advised that she was not too badly injured but had back pain and wanted to lay down. It appears no one else was standing or was injured from this incident. Our excellent team of flight attendants around her made sure she got ice; ibuprofen; and a row to lay down for the remainder of the flight. She advised medlink was not necessary. Around 60-90 minutes later; we were rotating pilots and the Captain returned to the flight deck. After the FO vacated to go on break and some discussion between the Captain and I; we called back to the aft galley; asking to see if we should call medlink on the injured FAs behalf. They called us back with her response saying 'maybe'; and out of an abundance of caution; we called via satphone. In short; Dispatch was on during the call and medlink advised that the actions already taken were appropriate (ice and ibuprofen); let her rest aslong as possible and if her condition worsens at all that we should call back. We arranged for medical checkout at the gate; and the situation did not seem to worsen. By the time the pilots could make our way back to check in with her after the passengers had deplaned; she was able to speak and walk to deplane on her own. The paramedics took basic vitals including blood pressure and heart rate. I stayed with the flight attendant; we were processed thru customs and went our separate ways curbside. The flight attendant was taken to hospital for check up by a family member; results as of writing this report are unknown.