Performance Analysis:
It just goes to show you how amazing Mariano Rivera‘s career has been, when he can post an ERA of 3.20 and those of us in the analysis community can all think something must be wrong. That is the scenario we are presented with though; despite striking out batters at his highest rate since 1996 (his first season as a full-time reliever) and handing out free passes less often than he ever has, Rivera is struggling relative to expectations.

The number in his performance so far this season that immediately jumps out is his home-run rate, which sits at 1.8 HR/9. You may think this is easily explained by his new digs, as Yankee Stadium II hasn’t exactly been on friendly terms with pitchers these past two months, but that’s not the case: Mo has three home runs allowed at home in 16 2/3 innings, and a pair on the road in about half as much work. Four of his five homers allowed have come on fly balls, though he isn’t giving up anymore of those than he usually does. Another source of worry is his BABIP, which sits at a career-high .336. It’s not necessarily the fault of the Yankees‘ defense-they are right around the league average in Defensive Efficiency. The problem might be better found in Rivera’s liner rate, which sits at 25.4 percent, nearly a double-digit increase over his career rate, and much higher than anything we have seen from him since this data was first recorded back in 2002. This also means that his ground-ball rate is at its lowest since that time, which isn’t what you want to see when the ball is leaving the yard this often.

Additional homers and plenty of line drives means that Rivera is throwing pitches that the opposition can hit, whether with his famous cutter or pitches identified as vanilla fastballs. Using published velocity data going back to 2002 up through 2008, courtesy of Fangraphs, Rivera has averaged at least 93 miles per hour on pitches described as pure fastballs and, at its lowest, 92.8 mph on those classified as cutters. However slender the real distinction between the two pitches may be, this year Rivera is at 91.6 and 91.2 mph; while it’s tough to pin an exact run value on that missing velocity, the drop does hint that those extra liners and home runs aren’t from mere luck. This also puts some context behind his falling infield fly rate, which went from last year’s impressive 24.5 percent down to his current 14.3; while many pitchers would love to get that many popups, for Rivera it contributes to why his HR/FB rate has jumped from 7.5 to nearly 24 percent.

Rivera is also throwing fewer first-pitch strikes; while 59.6 percent is still above the average, it’s below his career rate and his recent work by a few percentage points. He’s also generating fewer swings and misses-16 percent overall, and just 14 percent when he’s behind in the count. That’s a significant drop from the past two seasons, when he made opponents swing and miss on nearly one-quarter of his pitches, and even more than that while behind the hitter. Clearly he is still getting hitters to miss-look no further than his punchout rate to see that-but the pitches they’ve caught up to are being punished.

Given the things he is doing right, you want to say that this is a rough patch caused by a few bad appearances-the kind of thing we won’t even notice when the season is over and he has tacked another 70-plus innings onto his career. The things he is struggling in often end up as temporary setbacks as well-remember how awful Roy Oswalt was during the beginning of last year, when his velocity dropped and the homer rates skyrocketed? On the other hand though, Rivera is 39 years old now, and the loss of a few ticks on his famous cutter could pose a serious problem. Time will tell if these setbacks are the stuff of permanence, or if Rivera will be doing just what we expect him to from here on out. If it’s the former, remember: he’s still pretty good at this, even as a merely mortal closer.-Marc Normandin

Past Performance:
It was all unexpected and unplanned. Mariano Rivera became one of the greatest closers in baseball history by accident. Signed as a free agent out of Panama in 1990, the skinny right-hander was an intriguing starting prospect, posting low ERAs and high strikeout rates in the lower minor leagues. Baseball America named him the ninth-best prospect in the Yankees’ system; though he had a good fastball, Rivera was still developing his secondary pitches, and staying healthy was an issue. Dr. Frank Jobe performed surgery on Rivera’s right elbow, truncating his ’93 season, which was followed by shoulder soreness in ’94. After struggling to stick as a fifth starter for the Bombers in the first half of ’95, something changed. He dominated at Triple-A, the Yankees brass noticing that the 25-year-old’s fastball had suddenly ticked up to 95 miles per hour. Recalled to fill out the staff, he was put on the spot and shined during the American League Divisional Series against the Mariners; the next season, Joe Torre made Rivera his set-up man for closer John Wetteland. The secondary pitches mostly disappeared in favor of a cut fastball with devastating late movement, a pitch so good it propelled Rivera to fame and glory as baseball’s most dominant one-pitch pitcher.

Since then, even as Rivera ascended to closer and became one of the great post-season pitchers in history, the question has been how long a pitcher with just one offering can carry on? Rivera’s unique attributes make this a more difficult question than it would be for any other pitcher. One-trick ponies generally do not last, even the greatest of them, but Rivera will finish his career with close to 1,000 games pitched. Strikeout rate is one of the key indicator’s of a pitcher’s short- and long-term viability, but Rivera’s strikeout rates have been up and down-at this moment, when Rivera appears to be more hittable than ever, he is carrying the second-highest strikeout rate of his career. Rivera has also been unusual in that he’s a right-handed pitcher who owns left-handed hitters-thanks to the movement on that cut fastball, Rivera has held lefties to .208/.257/.263 rates on his career.

Where historical antecedents for long-term survivability are concerned, Rivera’s best hope may be the Hall of Fame reliever Hoyt Wilhelm. As with Rivera, Wilhelm threw a specialty pitch, the knuckleball, which depended on movement instead of velocity. Despite the movement of his flutterball, Wilhelm had good control and maintained high strikeout rates until the very end. Wilhelm was 39 in 1962, and was able to keep pitching for another ten years, posting a 2.15 ERA in 546 games comprising 974 innings. However, it is somewhat unlikely that Rivera can survive diminished velocity as well as a knuckleballer could, and diminished velocity seems to be Rivera’s problem this year.

Overall, there have been just five seasons of 30 or more saves recorded by pitchers Rivera’s age or older; two by Dennis Eckersley, two by Trevor Hoffman, and one each by Doug Jones and Todd Jones. With the exception of Doug Jones, who was dominant as a changeup-dependent 40-year-old Brewer in 1997, none of these pitchers were operating anywhere close to their peak forms in these late-career seasons. It should also be noted that even in Doug Jones’ case, his days as a full-time closer were numbered, ending midway through the next season. Rivera is very much in, if not uncharted territory, murky waters.-Steven Goldman

A version of this story originally appeared on ESPN Insider Insider.