Leaf, who compared Mayfield to himself in March, told radio station 92.3 The Fan in Cleveland on Friday that Mayfield has raised concerns during interviews.
"A good litmus test for me is always when I listen to radio interviews or TV interviews, I tend to close my eyes and just listen to the voice, and hear the answer. I always say, 'If it sounds like the 1998 version of Ryan Leaf, there's definitely a red flag that needs to be raised there," Leaf said.
Mayfield passed for 4,627 yards and accounted for 48 touchdowns with just six interceptions while leading Oklahoma to the College Football Playoff and won the Heisman Trophy last season.
While that that is impressive, Mayfield is also linked to some less-than-stellar moments.
Mayfield planted the Sooner flag at midfield after winning at Ohio State and taunted Kansas fans as well. Those actions were preceded by a Feb. 25 arrest in Fayetteville, Ark., on charges of public intoxication, disorderly conduct, resisting arrest and fleeing, which resulted in a plea deal.
"I think, probably, the comparison with me is more appropriate," Leaf told FOX Sports Radio in March. "The highly competitive, borderline arrogant, angry individual. ... The biggest thing for me will be how he deals with failure. That's where my downfall was, when things began to fall apart, how I was able to deal with that. When the media is on you, you play a bad game, your whole city is on you, that's where we'll see where Baker Mayfield is at. ... Right now there's no evidence to back up that when things get tough, he won't break."
Mayfield is projected to be a first-round draft pick. NFLDraftScout.com's Dane Brugler has the New York Jets taking Mayfield with the No. 3 overall pick in his latest mock draft. The draft is April 26-28 at ATT Stadium in Arlington, Texas.
Leaf retired four years after he was selected second overall by the San Diego Chargers in the 1998 NFL Draft. He played in three seasons after missing the 1999 campaign with a shoulder injury.
His teams went 4-17 and he completed just 48.4 percent of his passes while throwing for 14 touchdowns and 36 interceptions.
Leaf's short career was muddied by run-ins with teammates, management and media, as well as injuries, and his personal life has been marred by legal and drug issues.