TPC section 9.51 does not require the officer to specifically ask you for assistance. In addition to the defense of a third person statute, you can use 9.51(b). This allows any person to use force to make a lawful arrest, especially if there is no reasonable way to manifest your intentions first. The manifest your attentions means telling the person he is under arrest and for what. Given that a fight is going on at the time, I would say you cannot reasonably manifest your intentions.
So can you make the lawful arrest to begin with if the officer does not ask for help? Sure. Remember that any citizen has the authority under section 14.01 of the Code of Criminal Procedure to make an arrest for a felony or a breach of the peace that occurs in his presence or view. A fight with a police officer might be a felony (aggravated assault on a police officer) or it might be a misdemeanor (resisting arrest) but it is always going to be a breach of the peace.
I know how I will react and that is to jump into the fight, probably not using deadly force to start. But being a retired officer, I think I can still use my knowledge and skill to help subdue a person. Obviously, this depends on the situation and there are some where it will go right to shooting.
But I do not recommend you jumping in to help fight unless you know something about it to start. And if you are not medically or physically capable of that part, do not get yourself hurt trying it. Go ahead and draw, but please remember about the round going through the right person sometimes. be careful about getting to close so the BG can grab your firearm or knock it out of line too.
And if you do see something like this, please get someone to call it in to 911, either while you are trying to help the officer, which ever way you decide to help. Get the cavalry on the way. You will be pleasantly surprised how nice it is to hear sirens approaching when you really need them, and unpleasantly surprised how long that will seem to take.