Coming off a season in which Walker exhibited modest skills growth, hopes were high. When the Diamondbacks announced they’d be using a humidor for home games, expectations soared, perhaps too much considering Walker's checkered injury history. Sure enough, he left his third start early with the dreaded right forearm tightness. Early reports suggested the injury was minor, but those proved premature as Walker soon went under the knife for a torn UCL. The Tommy John surgery was deemed successful, and Walker embarked on a throwing program in late September. Since the procedure was done in April, Walker has a chance to return in the second half. As is always the case, this is barring a setback with his role and effectiveness up in the air. Walker’s pedigree is worth a roster spot in keeper and dynasty leagues. With pitching roster spots so valuable, he shouldn’t be drafted with 2019 in mind.

Acquired from the Mariners, Walker posted full-season bests in ERA (3.49), home-run rate (0.97 HR/9), strikeout rate (8.4 K/9) and groundball rate (48.9 percent). The flamethrower was limited, however, by a 3.5 BB/9 and a blister issue that hindered him early and cost him about a month. Poor grip may explain erratic velocity and dips in swinging strikes (8.6 percent) and first-pitch strikes (58.6 percent). He flashed brilliance, though, with a 3.32 second-half ERA and a 3.02 ERA in eight pairings with catcher Jeff Mathis, who may be the muse for his diverse arsenal. Also, the new humidor at Chase Field could help Walker's home struggles (1.39 HR/9 with a 4.18 ERA last year). Health and mechanical issues have delayed the former top prospect's growth, but the 25-year-old is trending upward a year removed from ankle surgery.

Walker started his age-23 season with quality starts in each of his first four turns, but he fell off-track and earned quality starts in just five of his last 21 starts (one of which only qualified because the runs were unearned). He had some massive performances in 2016, striking out exactly 11 batters on three separate occasions (and walking one or zero hitters in each), representing an all-or-nothing approach to the game log. Traded in the offseason to Arizona, Walker now faces an uphill battle, moving to a tougher ballpark and an organization that has not been earning merit badges for their work with pitchers recently. Additionally, Walker was making in-season mechanical changes as late as last September, and we get the recipe for a long road of disappointment for the once-promising starter. The stuff promises better things in the future, but there will likely be more growing pains before Walker is ready to make an impact on a start-by-start basis.

A monstrously bad start overshadowed a solid summer for Walker. He was popped for nine earned runs in his season debut, five in his next, and then after two good starts (one earned in 12.3 IP), he was smashed again for seven earned leaving him with an 8.74 ERA through five starts. That took him off the radar for just about everybody, at least as a premium fantasy option. He reeled off a 2.88 ERA in his next 11 starts with 69 K in 68.2 innings. From that point on his final 13 starts saw the normal ups and downs of a young, inexperienced pitcher. He’d have three good starts here, two duds there, en route to a 4.83 ERA in 78.1 innings. Home runs were the main problem behind his struggles. He had a 7.44 ERA in seven starts where allowed two-plus home runs and a 3.78 ERA in 22 starts with 0-1 home runs allowed. If loved him last year, you still should. He still has all the tools for a huge breakout.

The organization's top pitching prospect, Walker's ascension to the Seattle rotation did not go smoothly last season. He began the year on the disabled list with shoulder stiffness, missing the first three months. He then had to endure the wrath of manager Lloyd McClendon after an uninspiring start at Triple-A over the All-Star break. Consequently, the Mariners kept him at Tacoma longer than expected, and when he finally returned in late July he walked six and was demoted right back down after the game. He didn't make it back to Seattle until September when he worked out of the bullpen for three appearances before making two starts. He fared well in that stretch, though, posting a 1.96 ERA in 23 innings with 20 strikeouts and five walks. It's at least something to build on for 2015 when the 22-year-old will again compete for a rotation spot in spring training. Walker still has incredible upside, and last year's experience could serve him well this season.

After he rolled through Double-A and Triple-A last year, the Mariners gave their 21-year-old future ace a taste of the big leagues in September. Walker acquitted himself nicely across three starts with 12 strikeouts and four walks in 15 innings. The 6-foot-4 right-hander throws his four-seamer at 95 mph but hits the high 90s, pairing it with a low-90s cutter that keeps hitters off-balance. His offspeed stuff needs continued development, but he looks ready for the big-league rotation, which he appears likely to make coming out of spring training. It wouldn't hurt if he got a little more minor-league seasoning, but he should be in the rotation to stay by summer if he doesn't make the rotation out of spring training.

Walker had an inconsistent 2012 campaign, not extremely surprising for a hurler at the Double-A level who only turned 20 in August. After dominating the first two months of the season, Walker struggled in summer and late in the year, finishing with a bloated ERA. The most curious part of his numbers was the massive decrease in groundballs induced. Despite those concerns, he was still hitting 98 mph late in the season, and the Mariners liked the development of his breaking pitches too. It's too optimistic to expect the 6-foot-6 right-hander to make the Seattle squad out of spring training, but he's an elite talent who could see the bigs by the end of this year.

Named the organization's top prospect by Baseball Prospectus, Walker is a 6-foot-6 right-hander who could be about a year away from Seattle. The 19-year-old dominated at Low-A Clinton last season, striking out 113 in 96.2 innings with a 1.54 GO/AO and a .202 opponents' average. A 2010 supplemental pick out of high school, Walker was named the organization's Minor League Pitcher of the Year. Walker is just 19 and needs more seasoning, but he's not too far away. He could go to spring training in 2013 with a shot at a rotation spot. Keep an eye on his progress this season.