Hi and thank you for your question.
It certainly could be the starter, but it would not be my first check since it was working fine before the battery died.
I would suggest double checking your connections at the battery, and then the cable going from the solenoid to the starter, to make sure they are all clean and tight. If all the connections seem to be fine, Battery plus and minus, plus wire into the relay, starter cable out of the relay, starter cable at the starter motor, then we can check for volts.
Make sure the bike is in neutral. If the side stand is down, it will not crank if the bike is in gear. To be sure, make sure the bike is in neutral.
Turn the key on, confirm the neutral light is on, and with a test light hold the probe on the starter cable. Now, hit the starter button and see if the test light illuminates. If it does, you have current flow through the relay (aka solenoid). Now check the starter cable at the starter motor in the same manner. If you have power to the starter motor and it does not turn over, the starter has issues. You can take it apart and see if it just needs to have the brushes replaced, or if you have to replace the entire starter motor.