A lot has been made of Ricky Romero and his sinker during the past week. First, Brandon Morrow gives him a Brooks Baseball rundown of his decreased usage of the pitch last season. Jeff Sullivan of FanGraphs then showed how he really just had an uptick in sinkers during 2011, with '12 around '09 and '10 levels. Finally, on Wednesday, Romero threw mostly sinkers during his Spring Training start to try and regain the feel for the pitch.
As Sullivan pointed out, control was probably the biggest problem with his sinker. He saw a big drop in Strike%, the same percentage drop as his Zone%, going from 48.5% in '11 to 39.1% in '12. I'd say Zone% is even more important, since fastballs of either kind are not true "chase" pitches, at least not compared to curves, sliders, and changeups. Looking at heat maps, he missed with the pitch the same way each year, but he wasn't able to get the ball in the bottom half of the zone on his arm side. Until he throws the pitch for strikes again, it's not a viable weapon.
Another problem, which is definitely not exclusive to him, is the incorrect dependence on fastballs to LHH and RHH. Many pitchers who have both four and two-seamers tend to use their two-seam/sinker more against opposite-handed hitters more often, a big mistake. It is tougher for a hitter to get the barrel on a hard pitch coming into them, opposed to tailing away. Romero had a 65/8 4-seam/2-seam ratio against lefties last year, while his RHH ratio was 53/13. Righties were able to go the other way with the sinker, hitting .333 with a .463 SLG, opposed to a .308 AVG for lefties with no extra bases. The fear of hitting batters with arm-side two-seamers should not be such a deterrent from throwing the pitch, whether it's Romero or anyone else.
However, when someone has such a sharp cliff dive in performance, it's about more than just one type of pitch. His four-seamer accounts for over half his pitches, and the goofy platoon split rears its ugly head again. Righties hit the pitch well (.346 AVG, .523 SLG), but lefties, who he threw the pitch more often to, hit it even better (.368/.624). The movement on the pitch is also odd, dropping about an inch of vertical movement a year.
It would be great to see a Blue Jays fan's perspective here, but the movement could be explained by a slight grip problem. In my playing days (D2 Winona State University), I never felt comfortable throwing a four-seamer, as my middle finger always stayed on the ball a bit too long, causing a slight cutting motion. As we know, cutters have a bit of a reverse platoon split, which would help explain why lefties hit his main fastball so well. The lack of true "life" on his four-seamer also lessens the vertical difference between his two fastballs, rendering each of them less effective.
His changeup also showed some signs of decline last year. First, the pitch lost about an inch of fade and depth last year, though it is still very good. Second, the Contact% rose from 56.8% in '11 to 66.5% last year, again still very good. Hitters were also able to hit more line drives and flyballs on the pitch, which leads me to believe they could see it coming earlier.
First, there is the chance he is telegraphing his pitches. His release point was 3-4 inches lower for his fastballs than his offspeed, which is quite common. He also released his fastballs about 3-4 inches further on his arm side than his offspeed, which may be more of a tip. There is also a chance of other tips, such as glove fanning and double-patting.
Something even more likely for the ineffectiveness is the relation of spin between the two-seamer and change. With his decreased two-seam usage, it was more likely that a pitch with counter-clockwise spin was going to be a changeup. This also concurs with his 2010 season, when he threw less two-seamers and had a higher Contact% with his changeups. It doesn't matter how good a pitch is; if the hitter knows it's coming, they will handle it just fine.
Romero's disappointment last season was partially due to the very lucky 2.92 ERA from the year before. His xFIP was about 10% below average his first three years, so if you expect him to have a 3.60-3.80 ERA, you don't have ace expectations. He needs to find a pitch to help him against lefties, as his main offspeed pitches are his changeup and a good 12-6 curve, another near-neutral platoon pitch. He scrapped his slider after it was a main pitch for him in '09, so a two-seamer is needed to get lefties out. Without the ability to get both sides out, a starting pitcher won't last long in this league.