Before I get started here, I'd highly recommend reading Makakilo's article on why we will win the West next year if you haven't already. Good stuff, especially if you're into analytics and such.
So if you haven't heard, there's quite the hype train following the Diamondbacks around. Zack Greinke! Shelby Miller! Patrick Corbin! Paul Goldschmidt! A.J. Pollock! Those are all very solid reasons for some excitement. However, I'm afraid I'm here to run that hype train right off the rails.
Black Holes of Offense
Let's start off with Yasmany Tomas. After a decent start to his rookie year (.313/.358/.448 first half triple slash), he completely disappeared in the second half. Instead of improving after getting experience in the big leagues, he struck out in 35% of his 2nd half plate appearances. Tomas will need to make some drastic improvements to his offensive game if he wants to make a positive impact on the team. His defense was even more dismal. Just take a look at this quote from Fangraphs' profile of him: "He struggled to find a position, as the Diamondbacks (inexplicably) played him at third base for 272 painful innings, where the metrics and the eye tests agreed that he was terrible. He wasn't much better in the corner outfield spots, where the metrics said he was one of the worst outfielders in baseball." Not exactly an encouraging sign.
Now let's head over to the middle infield, where you can take your pick of a wide variety of bad to okay-ish offensive players. Here is the OPS+ from the last two years of the 4 most likely guys to get time in the middle infield:
Nick Ahmed: 66
Jean Segura: 69
Chris Owings: 72
Phil Gosselin: 105
The only guy on that list above league average is expected to have a bench role. You can talk about bounce-backs for Segura and Owings, you can talk about Ahmed's glove, but it seems that the Diamondbacks will be running out two players who are well below league average offensively in the middle infield. When you include Tomas in the everyday line-up, that is 3 spots filled with offensive black holes. It will be extremely difficult to win a division like that.
Regression from the stars
Zack Greinke is not going to have a 1.66 ERA again. Sorry. Looking at his career numbers, a low-to-mid 2 ERA would be a reasonable expectation. That's still fantastic, but will it be enough? Another concerning stat is that his ERA from 2004-2012 was a 3.77. He seems to have figured things out, with a below 3 ERA the last 3 years, but what if he goes back to that pre-2012 form?
Paul Goldschmidt may also regress a bit after a monster season last year Regression doesn't apply to Mr. Goldschmidt. I apologize for even suggesting it.
As for the rest of the core (Miller, Corbin, Pollock, Peralta), they will all need to be as good, if not better than they were last year. The supporting cast just isn't strong enough to make up for any decline in production from these guys.
Although he's not really a star, another key part of last year's offense who will regress is Welington Castillo. After coming over from Seattle in the Mark Trumbo trade at the beginning of June, Castillo hit 17 homers and had an OPS+ of 116. The huge offensive upgrade he gave the team at catcher was one of the main reasons the D-Backs were able to get to 79 wins. Sadly, it is highly unlikely he reaches this level of production again next year. Until 2015, he was always about a league average player offensively. I'm just not sold that the D-Backs will be able to score as many runs as last year if he hits league average.
The Dodgers and Giants will be very good
The only notable players the Dodgers lost from last year are Greinke and Jimmy Rollins. The Dodgers will now be starting top prospect Corey Seager at shortstop, who is a HUGE upgrade over Rollins. As for Greinke, yeah, that's a pretty big loss. However, the middle of their rotation was strengthened this offseason with the additions of Scott Kazmir and Japenese star Kenta Maeda. When Brett Anderson and Hyun-Jin Ryu return from injury later this season, they'll have a solid rotation from top the bottom. Since the team has changed so little, there is no reason that they won't be able to win 90+ games again in 2016.
Looking at the Giants lineup, the only real weak spot seems to be Angel Pagan. Everyone else in their starting lineup next year had an OPS+ higher than 100 last year, so it'll be very tough to get through the order. The additions of Johnny Cueto and the overpaid Jeff Samardzija give the rotation and upgrade over last year, where it was Madison Bumgarner and Some Other Guys. Oh yeah, even year magic blah blah blah. It is going to be a tight 3-team race all year in the West. The Diamondbacks just won't quite have enough to be able to pull away late in the season
The Diamondbacks need to win 13-16 more games this year
In the past 7 seasons, it has taken 92+ wins to win the division. Given the talent in the division, it could easily be more than that. Are the Diamondbacks improved enough to go from 79 to 92 wins? There's a chance, but a whole lot of things need to go right for the D-Backs.
To wrap it all up, the Diamondbacks' core players will not receive enough help from the supporting cast to win the division. On the offensive side, the middle infield and left field will all hit welllll below league average. If any of the star players falter (Goldy, Pollock, Peralta, Greinke, Miller, Corbin) or get injured, the D-Backs are not balanced enough to make up for that decline in production. In what looks to be a very competitive division, Arizona won't be able to get that 13 game improvement necessary to win the West.