Trevor Cahill, Diamondbacks
Cahill showed up to camp svelter than usual. The offseason work paid off with a strong April, as Cahill averaged more than seven innings per start while striking out about 2.5 batters per walk issued. He saved his best for last: throwing eight innings of one-run ball on Tuesday against the Giants. The bread-and-butter of Cahill's arsenal remains his sinker. His secondary pitch of choice has changed, however. Cahill threw his cutter 26 percent of the time in April, compared to 11 percent in 2012. Increased confidence in the pitch gives Cahill a fourth option, or at least a backup plan on nights when he cannot find the feel for his changeup.
Patrick Corbin, Diamondbacks
Yes, another Arizona starter acquired through an earlier trade. Corbin allowed one home run in 33 innings after allowing 14 homers last season in 107 innings. A considerable difference, and one that allows for improvement even after regression. There are two encouraging signs from Corbin so far: 1) his velocity is slightly up, and—more importantly—2) his command has been better. Corbin must stay down in the zone in order to be effective. He's done just that early this season.
Kevin Correia, Twins
When Correia signed a two-year deal everyone wondered who the Twins were bidding against. Through the season's first month Correia owns one of the weirdest lines in the league. Despite improved ERA and FIP measures, Correia's start feels unsustainable for his reliance upon his defense. Last season Correia averaged 4.7 strikeouts per nine; in April he averaged 4.9 strikeouts plus walks per nine. There's pitching to contact and then there's Correia.
Interestingly, Correia has shown more trust in his changeup. Throwing a changeup when trailing in the count is a savvy move—one that can result in weak contact—but one that requires conviction. So far, 36 percent of Correia's pitches to left-handed batters when down in the count have been changeups. His previous career-high is 19 percent. You wonder how much Rick Anderson has influenced Correia, and if trusting a group ranked 26th in defensive efficiency can end any way but poorly.
Hisashi Iwakuma, Mariners
When Iwakuma is on he grants hitters height or width but not both. His game depends on command over all his offerings, not just his fastball. Those offerings are not stellar on their own, but he makes the most of them by painting the borders of the strike zone. One intriguing subplot around Iwakuma is his tendency to exit games before hitting triple-digits pitch counts. Iwakuma hasn't flirted with 100 pitches yet, or even drawn suggestive circles on the back of its hand yet. Is Eric Wedge's relatively quick leash caused by Iwakuma's physical limitations—blisters have been cited—or is Wedge concerned about Iwakuma facing a lineup for a third or fourth time?
Kyle Kendrick, Phillies
The big statistical change for Kendrick revolves around his hit and home-run rates. The former can be explained to some degree by an improved defensive unit. Philadelphia's gloves ranked 22nd in defensive efficiency last season, but are 10th so far this year. Kendrick has changed his arsenal just a bit, as Bill Baer pointed out. A decrease in cutters and increase in sinkers may help Kendrick find more consistency if he has better command of the sinker, though the package remains that of a no. 4 or no. 5 starter.
Matt Moore, Rays
The scary thing about Moore's April is he appeared to still have room left to grow. Moore's curveball, always the best of his secondary offerings, is seeing an expanded role this season whenever he gets ahead in counts. But there are areas where work is required. The inconsistent command that plagued him during his minor-league days continues to creep into play. Of course his numbers so far—five starts, 32 innings, 13 hits, four runs, 38 strikeouts, and 15 walks—suggest he's doing just fine. It just feels like Moore may have another gear left—even if the numbers worsen when he shifts to it.