Memo to Patrick Corbin: Keep the Heater Down

Patrick Corbin proved several things in his first full season with the Arizona Diamondbacks' starting rotation. He proved his slider to be one of the game's most lethal put-away offerings, holding opponents to a .132/.197/.236 line and league-best strikeout rate of 55.2% with it. He also proved capable of being a workhorse starter, tallying a staff-high 208.3 innings, while making quick work of batters, throwing on average 3.58 pitchers per plate appearance -- the lowest mark among qualified left-handed starters last season.

Yet he also proved to be inconsistent over time. Prior to the All-Star break, the 24-year-old southpaw held true to a 2.35 ERA and 0.99 WHIP, ranking third and eighth-best, respectively, among qualifying starters. His second half of the season was a far cry from the first, however. Over seven post-All Star break outings, Corbin's ERA elavated to 5.19 and his WHIP to 1.50, enough for third and tenth worst, respectively, among qualifying rotation arms.

How did opponents tack on nearly three runs more every nine innings against the former second-round pick in the second half compared to his pre-All Star outings? More than anything else, they took advantage of his elavated fastball.

Opponents' SLG% vs. Corbin's fastball, pre-All Star break

Opponents' SLG% vs. Corbin's fastball, post-All Star break

When located in the upper half of the zone prior to the All-Star break, Corbin's heater limited opponents to a .241 average and .365 slugging percentage. And as we can see from the first picture, the portion of the zone in which batters did accumulate extra-base hits was relatively inconsequential. The second half depicts another story entirely, however, as opponents raked their way to a .392 average and .598 slugging percentage against Corbin's heater, each of which were well above the league average marks of .274 and .421, respectively.

Given that Corbin's fastball tops out at 92 MPH on average -- essentially the 91.9 MPH league-average mark for starters in 2013 -- we know he will not be able to pump fastballs up out of the zone to induce frequent swings and misses with it, as he induced a whiff rate of 20.9% on such offerings last season, which was the third-lowest rate among qualified southpaws. Instead, he must focus on keeping his fastball down in the zone, where he held batters to a .291 slugging percentage and induced a league-best 67.2% ground ball rate.

Corbin proved last season his stuff is front-line material, but opponents adjusted to his fastball when elevated following the All-Star break. If he can keep his heater in the lower half of the zone, he will find more success.