Heupel led Missouri-friggen Missouri to 14th in passing offense, scoring offense, and 4th in passing efficiency. He's taking over a team that usually has the most talent in their conference. Let's put this in perspective: Missouri was never in the top 50 in any offensive category in 2016. I feel like people don't realize what heupel did as an offensive coordinator.
Tremendous job as offensive coordinator. Those total numbers don't put into perspective the "rough" start they had. SC, Purdue, Auburn were their worst games. Once that schedule opened up, they were as good as any offense, especially in the passing game.
The question is in transferring from an offensive coordinator to head coach. Many people spoke highly of Frost's acumen to become a head coach while an offensive coordinator. As I put it several years ago here, "he speaks like a head coach". Frost is a good offensive coordinator that was made to be a head coach. Heupel may just be a great offensive coordinator. But, yes, he's definitely underestimated.