Didi Gregorius wasn't healthy enough for the 25-man roster on Opening Day 2013, and probably wouldn't quite have made it, if everyone was fully fit. However, it didn't take long for him both to reach full fitness, and the team to need his services. On April 16, Aaron Hill had to be placed on the disabled list with a broken hand, and Gregorius was recalled from Reno. He made his first start two days later, memorably homering off the very first pitch he saw as a Diamondback, in Yankee Stadium, and was the primary occupant of the position the rest of the way. Gregorius started 97 games, with the bulk of the remainder (47 starts) going to Cliff Pennington.
While both of those men should make the Opening Day roster in 2014, there's another credible alternative, in the shape of prospect Chris Owings, who hit .330 over 125 games for Reno. Admittedly, that was Reno, a very hitter-friendly environment, but even on the road in Triple-A, Owings' line was .311/.340/.436. Admittedly, those road games still include some parks that are even better for hitters, such as Albuquerque and Colorado Springs. But, overall, there's a reason why Owings was voted both Rookie of the Year and also the Most Valuable Player in the entire Pacific Coast League.
Owings got a September call-up, making his major-league debut off the bench on the 3rd. Perhaps because the team wasn't in a pennant race, he got a decent look down the stretch, starting the majority of games after his promotion. Going in to the winter, the general perspective was that either he or Gregorius would be traded during the off-season, to address the team's other needs. However, it was Adam Eaton and Tyler Skaggs who were the prospect dealt for Mark Trumbo, and at this point, it looks increasingly likely that both men will be Diamondbacks, at least to start 2014. Add Pennington, and you have a potentially very crowded shortstop position.
Strength: cannon for an arm
Weakness: struggles against left-handed pitching
Gregorius came to Arizona with the reputation of being an all-glove, no-hit shortstop. Neither part of that quite matched expectations. His range was generally good, and his arm was probably the most powerful on the D-backs infield. However, its accuracy left a bit to be desired, with the occasional bit of mental sloppiness. Among the 26 shortstops with 800+ innings, Gregorius was 22nd by fielding percentage, and although he recouped some of that with his arm, Gregorius still only ranked 17th by UZR/150. His error total would also likely have been higher, but for Gold Glove Goldie vacuuming up some errant throws on their far end.
However, against that, Didi's offense proved somewhat better than advertised, with a .704 OPS, against the MLB average at SS of .681, which was virtually identical to the figure he posted in the minors. As a player who was 23 for the entire year, and getting his first real taste of major-league pitching, there still seems some room for growth. However, the overall decent number hid a sharp split: Against left-handers, Gregorius hit at the Uecker Line, with an OPS of a mere .512, a split also reflected in his 2010 and 2011 minor-league numbers, though not in 2012. If that gap continues this year, it will be a significant area of concern.
Strength: offensive production
Weakness: poor plate discipline
Signed out of high-school, Owings is even younger still, and won't turn 23 until this August. Even outside of Reno, Owings had a decent bat, hitting .290 in 2012, between Visalia and Mobile, and also showed impressive power for the position, with 17 home-runs. The big question is his ability to take a base, with his Reno walk-rate coming in at just 3.8%, less than half the MLB average (7.9%). There was some signs of life after his promotion, Owings walking six times (once intentionally) in 61 PAs, but that's obviously a small sample-size. If that doesn't prove sustainable as he matures (again: he's young), his offensive value will take a knock.
One thing he does have going is a little more positional flexibility, though he still only has 14 games as second-base in his professional career. I do note three of those were during his September call-up, and he may see some work there in 2014. However, he, Aaron Hill and Martin Prado are all right-handed batters, so there would not appear to be any platoon advantage to be gained from this. Otherwise, Owings' defensive reputation is solid enough, getting good jumps on the ball, and with a decent arm. All indications are that he would be able to stick at shortstop, if that's the way the team wants to go.
Strength: all-round defensive ability
Weakness: can't hit, pseudo-switch hitter
Pennington's value to the Diamondbacks in 2013 was almost entirely with the glove: by bWAR he was barely above replacement level at the plate (0.3 oWAR), but his defense more than made up for it. His 1.8 dWAR was surpassed only by four shortstops anywhere, all of whom got a lot more playing time than Cliff's 96 games. Put another way, his UZR/150 at short was 20.4, second only to the Braves' Gold Glove winner Andrelton Simmons (23.9) among players with 400+ innings at the position. It's probably not stretching a point to claim that, given a full season, Pennington could be a legitimate Gold Glove candidate - if they actually gave it, purely based on defense.
The counterpoint is Pennington's very limited offense, with a wRC+ of 65. Nominally a switch-hitter, his production collapses when batting as a right-hander: his OPS against left-handers over his career is only .601, although last year, he did actually hit slightly better there, then when he batted as a southpaw. But over the past two seasons, one in Oakland, one in Arizona, Pennington has an overall line of .226/.290/.310. That .600 OPS ranks him 257th among the 260 players with 600 or more PAs over 2012-13. You're definitely relying on Cliff's glove for his value: any offense you get, is less pleasant surprise, and more of a stunning shock.
"Let them fight it out. We like both of them a lot. I love the competition in Spring Training... I want C.O. and Didi to be in the same situation. Just because Didi was there all year, I don't want him to think he's the everyday shortstop."
-- Kevin Towers
It looks like this one will be decided over the course of the next couple of months, though it's hard to say whether the team will be looking more for offense or defense. The most likely scenario seems to see one of them prevailing, with the loser returned to Reno, to get the everyday starts there, and Pennington operating as a backup. That could give Owings the edge, since he'd form a more natural platoon-mate with Pennington, both in handedness and their contrasting playing value. But Gregorius is the incumbent, and if Owings' walk-rate does prove to be a mirage, the fact Didi's weakness is against the less-common kind of pitchers could tip the balance in his favor.