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What Should the Diamondbacks Expect from Trevor Cahill? | December
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The Diamondbacks made a big trade during the Winter Meetings, trading top prospect RHP Jarrod Parker, along with OF Collin Cowgill and RP Ryan Cook, to the Oakland A's for pitchers RHP Trevor Cahill and LHP Craig Breslow. The Diamondbacks are trading a player with a lot of upside in Parker for the more established Major Leaguer in Cahill. But is important to note that while Cahill has already pitched for three years in the Majors, with 96 Games Started and 40 Wins, he is only 8 months older than Parker, who spend most of 2011 in AA Mobile. Cahill is already signed to a reasonable contract through 2015, so the Diamondbacks will be able to keep him for several years. But the big question is - how good is Trevor Cahill, and is he likely to be better than Jarrod Parker during that time?


Minor Leagues
Trevor Cahill was a 2nd Round pick by the Athletics out of Vista High School in Oceanside, CA. He spend 2007 and 2008 in the Minors, between Low A, High A, and AA, and was excellent at every level. His career record in the Minors was 22-9 with a 2.68 ERA, and he struck out 264 batters in 238.2 IP, for an excellent rate of 10.0 K/9 IP. He allowed only 163 Hits in those 238 IP, but did walk 97, for a walk rate of 3.66 BB/9.

 

Year Team W L ERA WHIP IP BB/9 K/9 HR/9
2006 Athletics (Rk) 0 0 3.00 1.000 9.0 7.0 11.0 0.0
2007 Kane County (A) 11 4 2.73 1.187 105.1 3.4 10.0 0.3
2008 Stockton (Hi A) 5 4 2.78 0.950 87.1 3.2 10.6 0.3
2008 Midland (AA) 6 1 2.19 1.162 37.0 4.6 8.0 0.5
TOTALS 22 9 2.68 1.089 238.2 3.7 10.0 0.3

Major Leagues
Heading into the 2009 season, Cahill was rated the #11 Prospect overall by Baseball America. He made the Major League club out of Spring Training, and has pitched for the A's for the last three seasons. The biggest difference for Cahill in the Majors was that his strikeout rate plunged from around 10.0K/9 all the way down to 4.5K/9 in 2009, although it has been climbing back up in the last two seasons.

Year W L ERA WHIP IP BB/9 K/9 HR/9 FIP bWAR fWAR
2009 10 13 4.63 1.438 178.2 3.6 4.5 1.4 5.33 1.9 0.6
2010 18 8 2.97 1.108 196.2 2.9 5.4 0.9 4.19 4.0 2.2
2011 12 14 4.16 1.425 207.2 3.6 6.4 0.8 4.10 3.5 2.5
Totals 40 35 3.91 1.322 583.0 3.3 5.5 1.0 4.51 9.4 5.3

Cahill has benefited from pitching in pitcher-friendly Oakland Coliseum, with a career Home ERA of 3.27 compared to a Road ERA of 4.71. He has been excellent against right-handed batters, holding them to a .244/.311/.364 (.675 OPS), while lefties have hit him a little better, at .261/.334/.430 (.763 OPS).

One reason why the Diamondbacks like Cahill is that he is an extreme ground ball pitcher, with a career 1.16 GB/FB ratio. His GB percentage of 53.3% is the 6th best in the Majors over the last three years, among pitchers with at least 500 IP. Although his career HR rate of 1.0/9 IP is not particularly outstanding, it has been improving each season. 

Pitcher Team GB%
Derek Lowe Braves 58.0%
Justin Masterson Multiple 56.4%
Fausto Carmona Indians 55.2%
Joel Piniero Multiple 55.2%
Ricky Romero Blue Jays 54.6%
Trevor Cahill Athletics 53.3%
John Lannan Nationals 52.5%
Felix Hernandez Mariners 52.5%
Rick Porcello Tigers 51.9%
Paul Maholm Pirates 51.1%
Roy Halladay Multiple 50.8%

(Pitchers ranked by Ground Ball %, from 2009-2011, minimum 500 IP)

Cahill made the All-Star team in 2010, and finished 9th in the Cy Young voting that year, with a 2.97 ERA.  His ERA was significantly better than his FIP in both 2009 and 2010, so is it possible that evaluating Cahill from his FIP and fWAR (FanGraphs' Wins Above Replacement) is underestimating his value? How did Cahill's ERA finish so far below his FIP that year? Primarily because of his .236 BABIP (Batting Average on Balls in Play), which was by far the best in the American League. His BABIP in 2009 was also good, at .272, but was actually higher than average in 2011 (.302). So was 2010 a fluke, and is Cahill more of a low-4 ERA pitcher, as his FIP predicts?  

One big change in 2010 was that Cahill started throwing his curveball a lot more, up from 2.7% in 2009 up to 13.1% in 2010. This was probably a big factor in Cahill's Ground Ball Rate taking a jump in 2010, and his BABIP decreasing.

Year Fastball Slider Curve Changeup LD% GB% FB%
2009 69.5% 8.4% 2.7% 19.4% 18.1% 47.8% 34.1%
2010 64.1% 4.6% 13.1% 18.3% 15.0% 56.0% 29.0%
2011 58.7% 6.6% 12.7% 22.0% 18.8% 55.9% 25.3%

And just as importantly, his location with his breaking pitches became much better. An excellent article by David Golebiewski at Baseball Analytics.org shows the pitch locations for Cahill's breaking pitches from 2009-2011. In 2009, he was throwing too many curves and sliders in the middle or upper part of the strike zone, and batters were hitting them. In 2010 and 2011, Cahill did a much better job of keeping these pitches low in the zone, and batters had a much harder time making contact with these pitches. Golebiewski's article includes "heat maps" showing pitch locations and contact rates for each year.

Year Lower Zone Middle Zone Upper Zone Swing & Miss
2009 50 29 21 22%
2010 52 27 21 35%
2011 58 22 20 36%

These stats can help explain Cahill's big improvement from 2009 to 2010. But he continued to improve the location of his breaking pitches in 2011, yet he gave up a lot more hits than the previous year. His Line Drive Allowed rate was an outstanding 15.0% in 2010, and increased to 18.8% in 2011. It could be that batters weren't prepared for Cahill's increased curve balls in 2010, and made poorer contact as a result. In 2011, even though he was still throwing the breaking pitches low in the zone, batters were ready and his BABIP went up. 

Conclusions
So what should we expect in 2012? Cahill is probably not going to pitch as well as his All-Star season in 2010. But his underlying stats have been trending upwards over his three years in the Majors. He strikeout rate has increased from 4.5K/9 up to 6.4K/9, and his Home Run rate has dropped from 1.36 HR/9 down to 0.82 HR/9. He's keeping a higher percentage of his breaking pitches low in the zone, and getting more batters to swing and miss. He is continuing to get a very high number of ground balls. He has shown excellent durability, with 583 IP over the last three years.

All of these factors explain the trade from the Diamondbacks' perspective. While many (including me) expect Jarrod Parker to be an excellent Major League pitcher, Cahill is more of a sure thing. He's been durable, and he's already proven at the Major League level. Cahill is still only 23 years old, so it wouldn't be surprising to see him continue to improve. I think a reasonable expectation for Cahill is around 200+ IP with an ERA right around 4.00. That should make him an excellent #3 starter for the Diamondbacks in 2012, behind Ian Kennedy and Daniel Hudson in the rotation.