Head injury
 

Who has the authority to deem a player back into the game after a head injury has been sustained? What if there is no certified athletic trainer on site?

At the very least a Doctor of Medicine or a Doctor of Ostepoathic Medicine. Some states may also allow the Head Coach or a Certified Athletic Trainer.

MTD, Sr.

This is probably a state-by-state thing.

In MN, we're told that, if we send a player out because of concussion-like symptoms, they cannot return until cleared by a "certified health care professional". Since we don't know who the "certified health care professional" in the gym is, if anyone, we're told that it's the coaches responsibility to ensure that their player has been cleared by said professional. If the player returns after being sent off, the officials are told to assume that the player has been cleared to return. (Of course, we can send them off again if we notice concussion-like symptoms again.)

each state has their own procedure for this. In IL, it's an MD or MO.

In Ohio, we have to see a form signed by an MD, DO or ATC. If they don't have one, he/she can't play.

Not just any head injury. There needs to be a sign of concussion before we step in and disallow participation.

Here in KS:

"after an apparent head injury or concussion the athlete must be immediately removed from
the competition or practice and cannot return to practice or competition until a Health Care Professional has evaluated
the athlete and provided a written authorization to return to practice and competition."

HCP defined as Medical Doctor (MD) or Doctor of Osteopathic
Medicine (DO)