Mariano Rivera back in pinstripes for another two years, there was one aspect of his 2010 stat line -- which overall, as always, was excellent -- that uncharacteristically raised a few eyebrows: his K/9 of 6.75, which was the third-lowest of his career. For a guy with a career 8.23 mark, and who fanned 9.77 men per nine innings in 2009, it seems somewhat curious that he would strike out significantly less hitters than Yankee fans have been accustomed to. Granted, not even Mo is immune to the effects of aging, but considering he's still performing at an elite level, I wanted to see if a deeper exploration of some of the numbers might guide us as to why Mariano punched fewer batters out in 2010.
Here's a chart of a handful of Mo's plate discipline percentages against since 2002, the first year we have data for:
The first thing that struck me was the spike in O-Swing%, up from 36.5% in 2009 to a career-high (well, at least since 2002) 40.1% in 2010. Mo's career number is 32.7%, although he's been above that figure every season since 2006. Still, that's a pretty significant increase in hitters swinging at non-strikes from Mo. Correspondingly, hitters also made more contact on those pitches, with a 75.7% O-Contact%, up from 2009's 73.3% (for additional context, Mo's career rate is 63.8%).
Mo also experienced a very small increase in overall Contact%, up 0.1 points to a career-high 83.0%. However, just two seasons ago it was 76.3%. Additionally, Mariano threw less pitches in the zone than he ever has before, with a 40.6% Zone% (down from 2009's 43.6%). And perhaps most tellingly, Mo threw a career-low 58.3% First-Pitch Strikes, which means he found himself working from behind more frequently than he ever had before. Though it's not depicted in the chart, hitters also jumped all over the offerings Mo did leave in the zone, swinging 67.4% of the time, up significantly from 61.3% in 2009.
As far as pitch selection goes, after throwing the Cutter a career-high 92.9% of the time in 2009 per Fangraphs, he used it slightly less in 2010 -- 84.9% -- and it also experienced a very slight (0.2 mph) decrease in velocity, but not by anything substantial, and it was still the best among AL relievers by far, at 16.6 runs above average. His fastball -- which he threw only 15.1% of the time (though apparently only 7.1% in 2009) -- actually had some extra zip on it, averaging 92.2 mph over 2009's 91.7 mph. However, his career fastball velocity is 93.3 mph, which confirms suspicions that Mo may have lost a tick or two off his heater.
Still, it's clear that diminished stuff is not the culprit with regards to Mo's declining K/9, but rather hitters taking a far more aggressive approach against Rivera than they ever have before and making contact with bad pitches at a higher rate than they ever have before. Mo also didn't do himself any favors by starting less hitters off with strikes than he ever has previously, which may have subsequently led to slightly less desirable pitch selection and perhaps to a lesser extent poorer pitch sequencing in some of those at-bats.
Ultimately I don't think there's anything in Mo's inflated plate discipline against percentages from 2010 that's cause for concern going forward, considering how far out of line they were with both his career rates and his numbers from one season prior, not to mention the fact that he still had another phenomenal year overall, but taken together they should at least partially explain why Mariano's K/9 fell by more than three strikeouts in 2010.