MLB.com earlier profiled five breakout hitters from 2016 and examined their chances of sustaining that improvement this coming season.
Now, it's time to switch the focus to five starting pitchers who will face a similar challenge in 2017:
Rick Porcello, RHP, Red Sox
2016: 22-4, 223 IP, 3.15 ERA (145 ERA+), 3.40 FIP, 1.3 BB/9, 7.6 K/9, 5.0 WAR
Repeat the feat: Although Porcello already had more than 200 big league starts before last season, he was also only 27 years old. The reigning American League Cy Young Award winner became a consistent force, with 26 quality starts out of 33 appearances and MLB's second-lowest walk rate. Porcello attacked opponents with five pitches, each of which he used at least 13 percent of the time, according to Statcast™. His arsenal includes a four-seam fastball that, thanks to a high spin rate, generated one of the best swing-and-miss rates in MLB last season, despite unexceptional velocity.
Pump the brakes: Porcello certainly pitched better than in 2015, when he posted a 4.92 ERA. However, he used a similar mix of pitches, generated a slightly lower ground-ball rate and saw only a modest uptick in strikeout percentage. On the other hand, he posted career lows in both his homers-per-fly-ball rate and BABIP -- which dropped all the way from .332 to .269, one of the 20 lowest marks in MLB. Porcello will have to prove he can continue to outpace his .307 career mark in that category.

Kyle Hendricks, RHP, Cubs
2016: 16-8, 190 IP, 2.13 ERA (188 ERA+), 3.20 FIP, 2.1 BB/9, 8.1 K/9, 5.0 WAR
Repeat the feat: Hendricks completed a short journey from marginal prospect to star in 2016, finishing third in the National League Cy Young Award voting. The 27-year-old doesn't have elite velocity or stuff, but he does have a lot going for him -- including an excellent, BABIP-suppressing Cubs defense. Hendricks induces plenty of ground balls, allowed one of MLB's lowest average exit velocities (87.2 mph) and finished fourth in terms of the lowest rate of barrels allowed per plate appearance (2.1 percent) in '16. He doesn't walk many batters and kept them off balance enough to lead MLB in called-strike percentage last season (minimum 2,000 pitches).
Pump the brakes: In terms of walks, strikeouts and home runs, there wasn't much difference between the 2015 version of Hendricks and the '16 version -- hence a FIP that dropped only slightly, from 3.36. One thing that did drop significantly was opponents' OPS with runners in scoring position -- all the way from .868 to .527. If that proves to be an unsustainable improvement, Hendricks won't post MLB's fourth-highest strand rate again.

Danny Duffy, LHP, Royals
2016: 12-3, 179 2/3 IP, 3.51 ERA (124 ERA+), 3.83 FIP, 2.1 BB/9, 9.4 K/9, 4.2 WAR
Repeat the feat: The best could still be ahead for Duffy, who spent the first month and a half last season in the bullpen before making a career-high 26 starts. As a starter, Duffy's strikeout rate of 25.4 percent and walk rate of 5.6 percent were huge improvements over his previous career totals in that role (18.0 percent and 10.1 percent, respectively), and he finished fifth in MLB in swinging-strike percentage (12.9 percent) -- just in front of Corey Kluber and Cole Hamels. In fact, the 35 missed swings Duffy got against the Rays in a 16-K performance on Aug. 1 tied Clayton Kershaw's Statcast-era record.
Pump the brakes: The talent is clearly there, which is why the Royals recently signed Duffy to a five-year extension. But the southpaw, who had Tommy John surgery in 2012, still has to show he can log a full healthy and productive season as a big league starter. After a stellar first few months in the rotation in '16, Duffy was hit hard over his last seven outings (6.37 ERA, 12 home runs).

Ivan Nova, RHP, Pirates
2016 (with Pirates): 5-2, 64 2/3 IP, 3.06 ERA (137 ERA+), 2.62 FIP, 0.4 BB/9, 7.2 K/9, 1.4 WAR
Repeat the feat: Nova was not particularly good for the Yankees from 2015-16, but the 30-year-old was also finding his footing again after '14 Tommy John surgery. So a change of scenery seemed to be in order. An Aug. 1 trade to Pittsburgh did the trick, and Nova eventually re-signed for three years. With the Yankees, Nova was last in the Majors in terms of throwing pitches within the strike zone, according to Statcast™. With the Pirates, he had the 21st-highest zone rate among 159 pitchers who threw at least 500 pitches over the final two months. That much more aggressive approach led to Nova walking just three batters in a Bucs uniform.
Pump the brakes: Stretched over a full campaign, Nova's BB/9 mark with the Pirates would have broken Carlos Silva's 2005 Major League record. Since that season, only two qualified pitchers have walked fewer than one batter per nine innings. In other words, it's highly unlikely Nova will avoid free passes that effectively in '17 -- especially if hitters adjust to his adjustment.

J.A. Happ, LHP, Blue Jays
2016: 20-4, 195 IP, 3.18 ERA (135 ERA+), 3.96 FIP, 2.8 BB/9, 7.5 K/9, 4.4 WAR
Repeat the feat: Happ already accomplished what Nova will now try to do, authoring a worthy follow-up to an excellent late-season stint in Pittsburgh. Something seems to have clicked for the veteran lefty, who posted a 4.73 ERA from the start of 2011 until his Trade Deadline move to Pittsburgh in '15 -- a period that included 50 starts in Toronto. Perhaps the secret is the movement on Happ's four- and two-seam fastballs, which he throws nearly three-quarters of the time, combined. According to Baseball Prospectus, the difference in vertical movement between the two offerings (more than 5 inches) was the widest gap among all qualified starters last season.
Pump the brakes: Happ almost certainly won't reach 20 wins again, coming off a year in which he got MLB's third-highest run support (6.3 runs per game). Meanwhile, the 34-year-old actually produced a lower strikeout-to-walk ratio and a higher FIP than in 2015, which could be troubling developments.