When will enough be enough for B.J. Penn?

More than 13 years have passed since Antonio Tarver knocked the legendary Roy Jones Jr. cold with a thudding left hand to the chin, and the image of Jones flopping around in the corner, attempting to get up, has yet to leave my mind.

Jones was one of the greatest fighters I ever saw, nearly unhittable at his peak, so fast, so athletic, so brilliant.

The bout with Tarver was only 14 months after the triumphant night in Las Vegas when Jones defeated John Ruiz to become the first former middleweight champion in more than 100 years to win a version of boxing’s heavyweight title.

The loss to Tarver seemed to signal the end of the line for Jones, though. Jones was 35 and was a certain first ballot Hall of Famer. There was nothing he needed to prove.

But 13 years, one month and two weeks later, Jones is closing in on 50 and is still an active fighter. He’s lost seven more times since that night, a few by violent knockout. No one knows if the end is in sight.

And that brings us to B.J. Penn, among the greatest mixed martial arts fighters to ever draw breath. For a brief period of time, nobody ever did it better than Penn, who was the second UFC fighter, following Randy Couture, to win titles in two weight classes.

Penn, 38, will fight Dennis Siver on Sunday at UFC Fight Night in Oklahoma City. Will I watch it? The better question is, can I watch it? To watch a once-great athlete like Penn struggling to compete with a mid-tier fighter like Siver isn’t particularly appealing.

I trust that Penn is healthy enough to compete, because UFC management has made that a hallmark of the way it does business. It’s not like Penn is showing signs of being punch drunk or of having taken too many shots.

But Penn isn’t the same fighter any more. He is only 1-6-1 dating back to April 10, 2010, in Abu Dhabi, when he lost the lightweight title to Frankie Edgar at UFC 112.

He’s been stopped in each of his last two bouts, including on Jan. 15, when he was overwhelmed by Yair Rodriguez.

Siver is a step down from Rodriguez, and from all of the opponents Penn has fought recently. Maybe that will make the difference.

In his prime, Penn was magnificent, an example of what a mixed martial artist could be. He was willing to fight anyone and his list of opponents reads like an MMA Hall of Fame.

He’s fought Matt Hughes and Frankie Edgar three times apiece. He’s met Georges St-Pierre and Jens Pulver twice. He’s faced Rory MacDonald and Jon Fitch and Nick Diaz and Sean Sherk and Matt Serra and Lyoto Machida and just about anybody who was a player in the early part of this century.

Seeing him now is like watching Willie Mays stumbling around for the Mets in the 1973 World Series, or watching Johnny Unitas getting trampled by the Pittsburgh Steelers’ Steel Curtain while quarterbacking the San Diego Chargers.

There is something different inside those who fight for a living. They’re willing to take risks that the rest of us can’t imagine taking. They torture their bodies to prepare for their bouts and they put their families through hell as they compete.

But they do it because they love it, and because there is something primal about challenging another man who has trained for months to beat you that appeals to them.

Some, like Penn and like Jones, never lose that desire, long after the skills that made them great have abandoned them. Penn believes he can beat Siver, and perhaps he will on Sunday.

But the B.J. Penn who helped push MMA to where it is today is long gone. The hero who submitted Matt Hughes is no longer among us. The spectacular athlete who was so quick, so precise and so focused that he battered Joe Stevenson into a bloody mess has been relegated to the history books.

That’s the B.J. Penn that I choose to remember.

This B.J. Penn, I wish the best. Win or lose, it doesn’t much matter, as long as he gets out without suffering injury and with his faculties intact.

The greats like Penn always believe they can summon up that special performance once again. It’s been instilled in them for decades that they’re more gifted than the average man and that they can do what few others can.

And so he fights, taking losses he’d never have taken in the past.

Watch if you must.

For me, I choose to remember the guy who won the welterweight title, who captured the lightweight belt and seemed like he’d hold it for a decade.

That’s the memory I want to hang onto, not one watching him being helped off the mat.