Every year, the Rays do this. They find a talented relief pitcher on the scrapheap and bring him into camp for close to nothing. More often than not, he goes on to have the best season of his career. So what should you expect from this year's edition, Juan "Leo Nunez" Oviedo? Take a look at his major league career (numbers from Fangraphs).
Oviedo has spent some time in the minors in each of his four years with the Royals, but those stints don't change the pattern. He was a pitcher with some good stuff but mediocre results and one year of fortuitous sequencing (2008) who figured something out when he went south to his new team. What did he figure?
Well, Oviedo throws three pitches (maybe four if you want to separate his fastballs into four-seamers and two-seamers, but I haven't taken a close look so I'm skipping that and lumping them all together). He throws a mid 90s fastball, a changeup, and a slider. Prior to 2009, he used his slider and his changeup pretty evenly. For instance, in 2008, according to the pitch f/x numbers on fangraphs, he threw his slider 17.2% of the time and his changeup 19.9% of the time. That all changed with the Marlins.
In 2009, Oviedo only threw a slider 7.5% of the time and he threw a changeup 34.6% of the time. His results were better, so in 2010 he upped the ante, throwing just 3.1% sliders and 35.3% changeups. Lo and behold, the results were great. He backed off in 2011, moving back to near his 2009 usage pattern and his 2009 level of results as well.
It wasn't just that Oviedo threw more changeups. They were also better. Consider these pretty pictures from Texas Leaguers.
Juan Oviedo's pitch movement in 2008:
Juan Oviedo's pitch movement in 2009:
There's a lot more drop on his changeup in 2009 (and every year since) than there was in 2008 (and every year previous). A straight changeup can be devastatingly effective when it hides within the fastball, but one that drops off the table can be used in more ways, and thrown more of the time. Oviedo's remade changeup isn't as good as Rodney's, but it's still a quality pitch, even in a vacuum.
Oviedo's 2012 was lost to legal troubles and Tommy John surgery, so he enters 2013 in the Rays camp and on a minor league contract. It's a pretty good landing spot for him. If there's one thing the Rays know how to do, it's handle pitchers who rely on the changeup. They fixed Fernando Rodney by (in part) having him drop his slider entirely and throw more changeups. If Oviedo's stuff is still good, he'll find a place in Tampa Bay.