Tom Arnold -- Offers to Mentor Johnny Manziel ... 'Let's Have Lunch' (VIDEO)

Tom Arnold knows firsthand about the dangers of alcohol -- and says he wants to speak with Johnny Manziel to make sure the QB doesn't make the same mistakes he did.

"I'd like to have lunch with him," Arnold said ... "He needs to get a mentor."

Arnold -- who's been very open about his own battle with substance abuse -- is clearly concerned about Manziel but says ... "maybe he's fine, but I'd love to sit down and talk with him."

As for Johnny -- he sought treatment for an undisclosed condition last year -- but says he's currently not drinking and "staying on track."