A lot of people thought this guy was going to be better than Peyton Manning. ESPN the magazine even had an entire edition dedicated to the debate, with some analysts picking "a little bit rock n roll" Leaf and others picking up "a little bit country" Manning.
https://splitcoverage.wordpress.com/2013/04/24/manning-vs-leaf-revisited/
His fall from grace was staggering, including addiction, being labeled the No. 1 draft bust ever, being arrested twice for stealing prescription drugs and being arrested again for busting parole.
Interestingly, his first game was against Buffalo. From Wikipedia: "The Chargers won the season opener on September 6, 1998, 16–14 over the Buffalo Bills despite mistakes from Leaf such as fumbling his first snap and throwing two interceptions; Buffalo penalties voided two would-be interceptions from Leaf."