Yes, you must call it on a specific player in this instance and yes, it would then also be an indirect on the head coach.
But you have some options here:
1)You can just choose one and wait until he protests and points to the real culprit
2)You can ask who did it, and if no one answers, threaten every player with a T for trying to influence an official's decision (yeah - this one goes over real well)
3)You can arbitrarily give it to the captain (although this is not the rule, either - and you cannot support it, but I have seen it happen)
4)My personal favorite - give it to the coach's kid
