Joel Carreno was signed by the Toronto Blue Jays as a free agent from the Dominican Republic back in 2004. He made his North American debut with a fine season for the Gulf Coast Blue Jays in 2007, posting a 2.62 ERA with a 64/13 K/BB in 65 innings, then continued to pitch well in the New York-Penn League for Auburn in 2008 (3.42 ERA, 85/19 K/BB in 76 innings). He remained effective for Low-A Lansing in 2009 (3.62 ERA, 62/29 K/BB in 80 innings) and especially High-A Dunedin in 2010 (3.73 ERA, 173/30 K/BB in 138 innings).Promoted to Double-A for 2011, Carreno had another fine season with a 3.41 ERA and a 152/68 K/BB in 135 innings for New Hampshire, with just 100 hits allowed. Jumped up to the major league staff in August, he's looked very good in his initial exposure, allowing just two runs in his first 13.2 innings with a 12/3 K/BB.
As you can see, Carreno has been solid at every level, posting excellent K/IP ratios in particular. Nevertheless, despite his performances he hasn't shown up highly on that many prospect lists. His best pitch is a plus breaking ball; it is so good that he actually uses it too much. The pitch is generally described as a slider by scouts, although pitch f/x identifies it as showing separation between a slider and curveball. There's nothing wrong with his fastball, a 91-94 MPH pitch with some movement, but he needs to show more confidence in it and has trouble commanding it at times. He has an adequate changeup, but for the most part the fastball/slider combination is his mainstay. His walk rate spiked in Double-A, but in the majors he's thrown strikes.
Carreno's arsenal and 6-0, 190 pound size give him classic projection as a reliever, but he performed well as a starter in the minors and might get a shot as a fifth starter at some point. At worst, he'll be a solid middle reliever who can be stretched into longer outings as needed. He could earn a significant role in the major league staff in 2012.