So far this year, Mo's peripheral rates are not as impressive as they were last year. His walk rate is up from 1.6 to 2.5, his strikeout rates has dropped from 9.8 to 7.4, and his groundball rate is down a bit, even though it is still solid at 48.9%. All of this accounts for a run and a half rise in xFIP, to 4.03. Though it would be nice to see the rates improve, I fully believe that Rivera is one of the few pitchers in the majors that has the ability to defy pitching metrics; his impeccable command would conceivably suppress batting average on balls in play --- this might explain his -0.75 ERA-xFIP differential since 2002.
Joe Girardi has said that Mariano has had a few lapses in command this year, explaining his poor performances against the Twins and Red Sox in the middle of the month. Sometimes it involves missing inside the strike zone, which led to the rare hard-hit balls he's suffered against him over the past month; sometimes it involves missing outside the zone, which goes with the increase in walk rate (Mo walked a batter and hit another yesterday).
In yesterday's game against the Orioles, Mariano was throwing as hard as he had in two years. He threw both types of fastballs at 95 mph, something we have seen extremely infrequently. Last year, he threw only one pitch that registered at 95 mph (94.5 on the potentially hot PITCHf/x gun); that was in California against the Angels last September. Before that, the hardest we had seen Mariano throw was in Boston in July 2008 (again, the Fenway PITCHf/x gun seemed to be running a bit fast then) when he threw both a cutter and a sinker at 96 mph in an at-bat to Mike Lowell.
Gameday PITCHf/x data is from MLB Advanced Media; it can be easily accessed via this tool.
Other stats are from Fangraphs.