Ken Holtzman was drafted by the Cubs in the fourth round of the very first MLB draft in 1965.
By September of that year, still just 19 years old, he was in the big leagues, making thre relief appearances for the Cubs.
And in 1966, with the Cubs retooling under Leo Durocher, Holtzman made 34 appearances (33 starts), with very good results for a 20-year-old: 3.79 ERA, 1.187 WHIP, 2.2 bWAR (not that anyone knew what WAR was in 1966). Nevertheless, he didn’t get a single vote for Rookie of the Year.
What Holtzman did have was a great fastball, and he threw several complete-game wins early in that season.