Black (away) player down and black regained ball. I whistled stop and accompanied injured player to sideline to tell coach there might be a concussion and therefore no reentry until medically cleared. As I spoke to the coach, the players resumed play. I turned around and whistled play to stop (again). White (home) coach started yelling that I should not be chatting with the coach while play was going on. He came on the pitch (about 2-3 feet) to continue arguing and I showed him a yellow card for coming on to the field without permission.