To date, Mariano Rivera has been pretty lights out for the Yankees. Not quite like he was last year; that was otherworldly. Still, save for a few rough outings, two of which came against the Rays, he’s locked down just about everything handed to him. Yet his numbers suggest that he’s lost a little something. Matthew Carruth of FanGraphs takes a look at what’s changed in 2009.

To sum up:

• Mo has a 7.5% swinging strike rate, the lowest of his career.
• He is throwing more pitches out of the zone.
• More pitches thrown out of the zone + fewer swings and misses = unsustainable strikeout rate (which is right around where it was last year).
• His home run rate is way up, but is unlikely to continue.
• His groundball rate is at its lowest ever.

Carruth doesn’t make any projections based on this data, but he does wonder: “which regression, the strikeout and walk rates or the home run rate, [will influence] Rivera’s final line the most.” It depends, really, on how you choose to look at the data.

Yes, Mo has a 7.5 swinging strike percentage, which is the lowest of his career. That is indisputable. However, it seems that this might be overblown a bit. He started off in April getting 10 swinging strikes on 148 pitches (6.7%). In May he increased that to 14 for 180 (7.8%). That has gone down slightly in June, but remember that June includes his sick day against the Rays. He’s currently at 7.6%, but if you’re kind enough to excuse the Rays appearance, in which he generated zero swings and misses, he’s at 9.1%. If we can accept that his illness was the reason for his performance that day, this is an encouraging trend.

As to him throwing more pitches out of the zone this year, it’s not by a huge margin. From 2006 through 2008, Mo has thrown 69%, 70%, and 69% of his pitches for strikes. This year it is at 67%. Again, let’s look to that Rays game. Mo threw 21 pitches, 10 of which were strikes. Remove those and his strike percentage is up to 68%. Also, it’s not like Mo hasn’t gone seasons at a 67 or 68 percent strike rate. I don’t think this is a big deal at all, especially considering the anomalous Rays appearance.

This speaks to the strikeout rate argument. Again, if Mo isn’t really deviating from his career norms in strike percentage, and if his swing and miss percentage, outside of the Rays appearance, is on the rise, this doesn’t seem to be much of a concern. Maybe he won’t sustain the same rate as last year, but that’s again not a huge concern because last year’s was the highest of his career outside of 1996. In 2006, when he had a 1.80 ERA, he struck out 18.8 percent of hitters.

To the home run rate, I’m obviously going to agree with Carruth. He has allowed just one homer, a meaningless bomb in a blowout of the Orioles, since allowing back to back shots against the Rays on May 7. Mo has also allowed a homer on 15.2 percent of fly balls hit off him, which is more than double the career high (outside of ’95) he set last year.

The final bullet point might be troubling. Groundballs and strikeouts are a pitcher’s best friend. Mo is getting his strikeouts, but apparently his groundball rate is a tad on the low end. As in, his groundball to flyball ratio, not to be confused with groundout to airout ratio, is 0.90. Mo hasn’t been that low since his injury-shortened 2002 season, and hasn’t had a full season at that level since 2000. This does seem troubling. Not even removing the Rays outing helps mitigate that stat.

On the whole, we shouldn’t be concerned about Mo. He’s pitched damn well since getting shelled against Tampa Bay on May 7. Outside, of course, his sick day on June 6. If you’re willing to forgive that, he’s mostly on track. Even if you’re not, the trend is encouraging. The only real area where he’s slacking is in groundball outs, and that could be a real concern. We’ll have to see that one play out. Otherwise, we can sit back and enjoy the Mo.