I'll offer a slightly differing opinion (and this is more opinion than directed practice). If the player is laying on the ground, and somewhat immobile, I'm not gonna restart the game until they can get him back in the team area (at least 12' off the sideline, between the 25's. The player and the trainers are at definite risk if action should come their way, and he can't get himself out of the way quickly. I will ask if he can be moved back into the team area. If so, and they do, then we get moving. If not, we wait until they can get him moved. Unless he is in the true team area, I will encourage them to get him moved to the team area, or completely off the field (like, back to the dressing room). If he has a hangnail, and can move himself out of the way, okay, 6' off the sideline is enough.
Robert