I think his numbers are actually pretty decent considering he skipped right over AAA and straight to ML from AA. The best comparison I have at my ML level is this guy:
Matt Hearn
It's a rough comparison, but the closest I've got to Leverton. Hearn's overall rating is slightly higher, as are his stamina and durability, but not by a whole lot. The velocity is the biggest difference, as Hearn is much higher there. And he's got a better GB/FB too. But Leverton has better control and vL/vR splits. Yes, there are a lot of differences between these two guys, but it's a rough comparison like I said.
Leverton's ML ERA is almost the same as Hearn's, but Hearn has a few more years of ML experience. Hearn is getting lit up pretty consistently this season, but I'm in a hitter friendly park (2, 0, 2, 1, 1). The two seasons before I took over, he was in a pitcher's park (-2, -2, -1, -1, -1), and before that, in an even more pitcher friendly park (-2, -2, -3, -2, -2). Last season, in Hearn's first season in a hitter's park, after 5 seasons in the pitcher's parks, he put up numbers consistent with his prior stats, which surprised me considering the 180 degree change in BP factor. But his numbers this year are much much worse.
All that being said, I realize I don't really have a suggestion. This is only my 2nd season of HBD so you know more of what to expect than I do. lol