The reason, which has nothing to do with the combine, should surprise no one.
"I don't know yet," Mayock said. "I take (Winston) No. 1 from a talent perspective. I'm all over it from a talent perspective, but I would have to do more homework off the field because, right now, he scares me. He was the face of Florida State football, and he continued to make bad decisions off the field."
Winston -- whose Pro Day is March 31 -- threw for 3,907 yards and 25 TDs during the 2014 season. But he also tossed 18 interceptions, tied for second-most in the nation, and that also gives Mayock pause.
"I think Jameis throws way too many interceptions," Mayock said on the show. "Obviously, he has the ability to close games. ... He was at his best when the best was needed and I give him credit for that. But he threw (a combined) seven interceptions vs. Louisville and Florida, and it could have been 14 if the other team could catch the ball. He throws the ball up for grabs, in my opinion, way too often. That's my big issue with him on the field."
Mayock said the off-field issues are a separate point.
"I talked to some psychologists in Indy that worked for teams," he said. "How much pressure does it put on a psychologist when the football guys are all going, 'Hey, we're good with him. Just bless him and let's go.' So, to me, that's the bigger issue: Do you believe this kid won't be Johnny Manziel, he won't be JaMarcus Russell? You got to believe in the kid first before you believe in the football."
As for Mariota, Mayock reiterated a point he made in a teleconference before the combine: Mariota hasn't run anything close to an NFL offense.
"He's the classic case of 'OK, all these individual components work. How much do we believe that he can transition into the role of NFL pocket passer?'" Mayock said. "Since there's very minimal proof of it on tape, it's a leap of faith. And I think most people are going to take that leap because the kid is so solid, so smart and so hardworking, they believe he'll be able to do it.
"My concern, again, is that most of that pocket awareness feel is innate. I think you're born with it or you're not. So, it's a little bit of a projection. I love the kid, and I think both of them will certainly be gone in the first six picks and probably in the first two."
Mayock touted Mariota's intangibles as a big plus.
"Everybody I've talked to, from (UCLA coach) Jim Mora, who competed against him every year, to (Chicago Bears guard) Kyle Long, who was his teammate at Oregon, says he is the ultimate warrior, the best leader they've ever been around, and even though he doesn't manifest it as vocally as, say, a Jameis Winston, he's nonetheless as good a leader," Mayock said.