Heupel was already outstanding when he came to the Sooners in 1999. I remember following all this on the Internet of 20 years ago and I knew a lot of Sooners fans on AOL. They were excited to get him because he was the top QB out of the JUCO ranks. This was right after Michael Bishop's career at Kansas State, where he'd come out of JUCO and made them a national title contender. Oklahoma was a bad team, but had talent and now it had a coach and a quarterback. They went a very competitive 7-5 in his first season. Some of their fans used the same line I see from UT fans now--"Believe the Heup!" The first time I saw it was before he'd even played a game for the Sooners.
As others have said, Mike Leach and Mark Mangino were both on that staff. But Heupel knew his stuff already. I saw some of his '99 games. He was what you might expect, a smart as hell guy with an average at best throwing arm, couldn't run well, but made good decisions and was accurate. He was a coach on the field (and a coach's son) long before he was ever a coach himself. If you had told me in 2000 that he would one day be head coach at UT, I wouldn't have thought you were nuts.
I pulled for the Sooners to beat Florida State in that national title game (I was sick of their excuses about losing the Fiesta Bowl to us), and I thought if they were going to give the Hypesman to a quarterback, Heup should have gotten it.
It's worth noting that OU has had better talent overall and better NFL prospects at quarterback many times since Heupel graduated 21 years ago, but they have not won another national championship since.
I really don't worry about the quarterback position as long as he's UT's coach. After making himself a star QB in college football, and once converting a wide receiver back to quarterback as a coach at OU and winning the Big 12 with him, he probably looks at a dude like Hendon Hooker and thinks that's child's play.