Crew was backfilling between sheet pile and new concrete wall. I was on the excavator dropping dirt into the space, Bruce Willis was in the space directing me, 2 Labor Finders employees were following us spreading the dirt with shovels. Bruce noticed that one of the guys was kneeling down and appeared to be in pain. He asked him if he was OK and he responded that a rock rolled off of the roadway edge over the sheet pile and hit him in the back. While spreading dirt he was in a bent over position, from the dirt he was standing on to the top of sheet pile were the rock rolled from is 4’ 8”. Bruce nor I actually saw it happen as we were focused on our activity at the moment. He stood up and continued to spread dirt. As about 30 minutes went by he stated that it was starting to get painful and that he wanted to seek medical attention. At that time James Watford was notified and Sandy from Labor Finders was also notified to arrange medical treatment. The rock that struck him was not able to be located, believed to be buried.