These are the kind of nights that make boxing both a mystery to its advocates and a menace to its practitioners. It is also the kind of night that makes it ghoulishly fascinating for everyone involved.

Tonight Israel Vazquez, as brave a warrior as there is in boxing, returns to the ring after a 19-month layoff. No one knows what to expect.

Vazquez has not fought since winning a split decision in the third of his bloody trilogy with Rafael Marquez on March 1, 2008, the conclusion of a three-fight bruise-fest that was both compelling to watch and alarming to endure even as a spectator. Since then, he has had surgery three times on his right eye to fix a detached retina.

He is scheduled to headline a Golden Boy Promotions show at the Nokia Theatre in Los Angeles against a journeyman named Angel Antonio Priolo. There is little reason to expect much from Priolo (30-7, 20 KO), who has not been summoned there to make a statement about himself, but there was little reason to expect much from Alfonso Gomez the night he beat the stuffing out of a well shot Arturo Gatti either.

This fight is not about Priolo in any way, even if he happens to win. It is about Israel Vazquez and what he has left inside…or doesn’t have.

Priolo was chosen precisely because Golden Boy matchmaker Eric Gomez has no more idea what is left of Vazquez (43-4) than anyone else does. All he knows is that fights like the three the former World Boxing Council super bantamweight champion, whose title was stripped because of injury-induced inactivity, endured against Marquez chip pieces off the edges of a fighter that cannot be glued back on.

This is unavoidable if one has a long boxing career but Vazquez should be, in theory, still a young man. Chronologically he is but in fighter years he aged plenty in the 25 rounds he shared with Marquez. How much is the question that will be explored and exposed by Priolo.

Vazquez’s blood, like Marquez’s, was everywhere the nights they fought. His eyes, like Marquez’s, were badly bruised and swollen half shut each time. His body, like Marquez’s, was asked to absorb the kind of punishment that doesn’t heal itself simply because some referee raises your hand.

Marquez himself recently returned to the ring and the jury is out if he is still the same fighter he once was. Now it is Israel Vazquez’s turn to face that test while knowing not even he has any idea if he can still pass it.

It seems unlikely Priolo can find any way to beat Vazquez, even in a dissipated state, but this is not simply about winning a fight. It would be a disaster for Vazquez to lose but winning alone may not be enough any more. This is about how much fight he has left in him.

In Gatti’s case, the years, the noble way he fought and won on nights when it seemed his body could take no more and finally the way Oscar De La Hoya and Floyd Mayweather, Jr. gave him beatings he could not survive, wiped out all that had once made him great. It left behind an empty shell for Alfonso Gomez to expose.

By the time he got in with Gomez, Gatti was like an aged, moss-covered stone wall in a New England woods, weakened by too many rough winters and by years of disrepair until all it took to make it collapse was one person leaning on it.

Gomez was that person. Saturday night we find out if Angel Antonio Priolo is that person for Vazquez or if he is proof that there is still something left inside a man who has always given all he had in the blind pursuit of glory inside the boxing ring.

If Vazquez wins, regardless of how it happens, he will very likely end up in a fight with Juan Manuel Lopez, the World Boxing Organization title holder fighting the same night in the WaMu Theatre inside Madison Square Garden 3000 miles away. The telling difference between them is that Lopez is 24 and undefeated, not having yet felt what Vazquez long ago chose to accept as part of the necessities of his dark trade.

For many, that fight will be seen as the true test but Saturday night is a pop quiz for Israel Vazquez and the more he’s popped the more difficult it will be for him to find the right answers ever again inside a boxing ring.

Maybe he will look as he did those last two nights against Marquez, when he stopped him in the sixth round and won a disputed split decision on nights where his will exceeded his skill. Then again, maybe he will look like a ghost, a shadow of who and what he used to be.

That is the painful truth about what will go on at the Nokia Theatre. It is what repels us and fascinates us at the same time. It is a chapter in a fighter’s story that has been written many times but continues to hold our interest for two reasons: we all love a comeback and we’re all fascinated by a train wreck.