It is becoming increasingly apparent that we are finally seeing the aging of Mariano Rivera. The great Yankee closer earned his 23rd save on Saturday, but it took a lot of work. While he struck out two, he brought the tying run to the plate (twice) by giving up two hits. Most significantly, he threw 28 pitches. This is the seventh time in 37 (18.9%) appearances he's thrown 20+ pitches in part, because batters are now able to work Mariano more effectively.
2009 Mariano vs. 2011 Mariano
As broadcasters repeat ad nauseum, Rivera is (primarily) a one pitch pitcher; he throws the cutter.
Here is the Mariano cutter in the first-half of 2009
Through July 14 of 2009, Mariano faced 119 batters, walked one and struck out 38. Batters hit .178, slugged .280, and had a .465 OPS. He had a 31.9% strikeout rate.
Here is the Mariano cutter in the first-half of 2011
In the first half of 2011, Mariano faced 106 batters, walked three and struck out 25. Batters hit .214, slugged .252, and had a .488 OPS. He had a 23.6% strikeout rate.
Beyond the slight drift upward of the pitch and the slight drift upward of the stats, there is a slight drift upward in the swing rate (49.5% to 53%), and a slight decrease in the miss rate (17.2% to 15.3%). Beyond that, there is an increase in the play rate 33.5% to 36.3%
2009 Mariano vs. 2011 Mariano against lefties
The difference is even greater versus lefties, take a look.
Here is the Mariano cutter in the first-half of 2009 against lefties
You can see that Mariano primarily worked inside, but he still worked enough on the outside to throw batters off.
Here is the Mariano cutter in the first-half of 2011 against lefties
The difference isn't vastly significant, but enough so that you can see why lefties are having more success this season. The pitches that Mariano used to throw on the outside are now being thrown more frequently inside and high.
Mariano will undoubtedly get the 20 saves he now needs to be the all-time saves leader, but he is going to have to work harder and his decreasing save percentage rate (98%, 96%, 87% to 85% this season), will most likely continue.