Air carrier First Officer reported uncommanded aircraft movement when the pushback crew disconnected the aircraft. The Captain stopped the aircraft abruptly which resulted in injuring a Flight Attendant.

Narrative

As we were pushing off XX; my captain advised me to start engine #2. As the engine was starting; the tug crew disconnected from the aircraft without my captain advising them the brake was set; clear to disconnect; etc. so the aircraft began to roll toward the ground crew. In an attempt to not run over the ground crew my captain used the handbrake to bring us to a stop; which jolted the aircraft and resulted in the lead FA getting knocked over and hitting her head. The ground crew quickly left the scene and was replaced by a new ground crew; not providing much communication to my captain about what had happened. We went back to the gate for her to get checked out by EMT's. She ended up being okay and we continued the flight with no other issues. Suggestions: Ground crews should be in constant communication with us to be sure we are all on the same page. My captain is adamant he didn't hear a request to set the brake; and i personally did not hear him verbalize brake set; clear to disconnect; good day" at any point prior to ground crew disconnecting and letting the plane roll forward."