BJ Penn is getting a SERIOUS offer from the man he's supposed to fight in October -- with Nik Lentz telling TMZ Sports he wants Penn to come live and train with him to help get his life back on track.

Remember, Dana White said Penn's last ever UFC fight would be against Lentz later this year -- but after Penn was KO'd during a street fight in Hawaii, it seems the bout could be in jeopardy.

Lentz tells us he "needs" this fight to happen for the sake of his own family -- but also he cares about Penn and wants to help him get cleaned up.

"Clearly, there's something wrong here," Lentz says about Penn's recent altercations ... "You need to get your life together."

Lentz says he wants Penn to move out of Hawaii -- where he thinks Penn is running with the wrong crowd and it's dragging him down.

"There's people there that are not good for you."

Instead, Lentz says Penn should relocate to Florida -- where he can train at his gym and sleep in his home.

"I'm gonna teach you how to be a man because right now you're a monster" ... adding, "You can't be getting knocked out in front of the Lava House."

But, don't mistake Lentz's kindness for weakness -- Nik says he won't have mercy with Penn when they step into the Octagon.

"I'm going to help him more than he's ever been helped in his life," Lentz says ... "I'm gonna kick his ass for money at the same time, but I'm still gonna help him."