We talked to Stallone -- a HUGE boxing fan -- outside Cafe Roma in Beverly Hills ... and asked the 'Rocky' star whether McGregor should take the bait and fight the YouTuber turned boxing star.
"Are you kidding. With the kind of money that's going around, he'd be foolish not to. Think about it."
Of course, Conor and Jake have been verbally sparring on social media and in the media .