Tom Griffin. Larry Dierker. Jordan Lyles?â†µ
The Astros rookie had a fair season, but he did it as a 20-year old. That's the best part, not his 2-8 record, his 5.28 ERA or his 67 strikeouts in 94 innings. He's got the second-most innings of all-time by an Astros pitcher making his debut at 21 or younger, behind only Griffin's 188 innings at 21. He's also the youngest pitcher to debut since Jeff Juden in '91. Only Dierker pitched more innings as a 20-year old.
The problem, again, is that Lyles wasn't particularly effective in his 94 innings. He was good, and impressive for his age, but not the kind of season that would make him a budding star. The comparisons I've heard are to Jeff Suppan and Brad Radke. Suppan debuted at 20, but didn't pitch more than 90 innings in a season until he was 22. His ERA was over 5.00 for the first four years of his career and didn't drop below 4.00 until he was 30 with St. Louis.â†µ
Radke debuted at 22 with a 5.32 ERA in 181 innings and finished ninth in the Rookie of the Year voting. His ERA dropped almost a full run in his next season and stayed around 4.00 for the following four years. He finished his career with 148 wins and a 4.22 ERA.â†µ
Of those two, I'd take a Radke career for Lyles.â†µ