FIVE-time Major winner Phil Mickelson has issued a stern warning to Aussie youngster Ryan Ruffels on the eve of his profession debut on US PGA Tour debut.

The 17-year-old made his US PGA Tour debut at the Canadian Open last year but will earn his first start as a pro at Farmers Insurance Open on Friday at Torrey Pines.

After controversy erupted with Ruffels’ revelation he won $5,000 from beating lefty in a social game, Mickelson said the former amateur star had “some things to learn” now he’s in the paid ranks.

“He’s a very nice person. I met him a month ago, and we played for the first time. He’s young, and he’s got some things to learn,” said Mickelson.

“One of them is you don’t discuss certain things. You don’t discuss specifics of what you play for. And you certainly don’t embellish and create a false amount just for your own benefit.

“So those things right there are, that’s high school stuff, and he’s going to have to stop doing that now that he’s out on the PGA Tour.”