It wasn’t supposed to be this way.

After yet another spending spree that included a $147 million splurge on Zack Greinke, the Dodgers were expected to be leading contenders for the NL West crown. Their chief competition figured to be the Giants, fresh off winning their second World Series in three years. The Diamondbacks were considered light sleepers, likely weakening their team after trading away Justin Upton for no good reason but still wielding a solid collection of young, homegrown talent. As for the Rockies and Padres? You figured wait till next year before this year even started.

So much for predictions. Only the AL East has seen more parity than the NL West, a division that has its top four teams separated by just 3½ games. Arizona leads the way, the Rockies and Padres are contending, with the Giants and Dodgers bringing up the rear. With so many teams bunched so closely together, the division up for grabs, and two of the National League’s three biggest spenders likely to double down, we could see a flurry of action as the trade deadline approaches.

ARIZONA DIAMONDBACKS

It boils down to the bullpen.

GRANTLAND’S TRADE DEADLINE PREVIEW • NL Central • NL East

Diamondbacks relievers rank a strong sixth in ERA and third in xFIP. But the D-backs have blown a staggering number of leads in the late innings, thanks largely to repeated blowups by their closers. Only the Dodgers and Orioles have blown more save chances this year than Arizona. Opening Day closer J.J. Putz blew four of nine save chances before hitting the disabled list with an elbow injury. Spotting for both Putz and his replacement, Heath Bell, when not performing setup duties, David Hernandez has racked up four blown saves of his own. Bell’s save percentage isn’t that bad (13 out of 16) and his peripherals look decent, led by a stellar 24.2 percent strikeout rate. But watch a few Diamondbacks games and you’ll see why #HeathBellExperience has become a go-to snarktag on Twitter: Bell has been grooving fat pitches all year long, and hitters are crushing them all over the ballpark. Putz is expected back this week. And the pen has received surprisingly strong contributions from Josh Collmenter, Will Harris, and Matt Reynolds, along with the usual cruelty to worms from Brad Ziegler.

So the D-backs have options. They could drop Bell to lower-leverage work until he gets sorted out — and a report Tuesday surmised that he’d found a mechanical flaw that might’ve caused all this carnage. You could then have Putz step back into the closer role immediately upon his return, have Hernandez get a shot, or (gasp!) use various relievers based on skill sets and matchups rather than blindly adhering to traditional roles. Since that last option’s likely to remain little more than a quixotic quest from fans of analytics/common sense, the other option could be a trade. Even if some teams sit on the fence about whether or not to become deadline sellers, cashing in a veteran reliever for help elsewhere is something any club could do without damaging the core of their rosters. Even if the D-backs prefer to focus on known commodities or even “proven closers”, there should be plenty of options.

Bullpen aside, the Diamondbacks should be in good shape, thanks to improved health. Aaron Hill just returned from the DL to reclaim his starting second-base job. Eric Chavez is expected back in a few days to bolster the bench and spot-start at third. If Adam Eaton makes it back sometime next month as hoped, Arizona would have a surplus of capable outfielders and thus trade bait for pen help or anything else they might desire. One commodity they likely won’t need: starting pitching. Even with Brandon McCarthy and Daniel Hudson out, Arizona’s collected so much good young pitching that they have strong options who can’t even crack the big league rotation right now, led by Tyler Skaggs in Triple-A.

A lot could change between now and August 1. But you should feel optimistic about the Diamondbacks’ second-half chances for the same reasons they’ve fared well in the first half: They’re good, and they’re deep.

COLORADO ROCKIES

Let’s start here: If Roy Oswalt’s the answer, you’re asking the wrong question. The 35-year-old right-hander is years past his prime, can’t stay healthy, and we’re now supposed to assume he’ll be effective for 15-plus starts, while making half of them at Coors Field?

Thing is, the rest of the Rockies’ starting rotation has ranged from competent to stellar. Though the performances of the top two starters, Jhoulys Chacin and Jorge de la Rosa, might seem a huge surprise, remember that both missed most of last season, and that Colorado’s huge pitching failures owed far more to a rash of injuries than a fatal lack of talent. With Tyler Chatwood showing vastly improved command and a 60 percent groundball rate through his first eight starts this year, the Rockies may well go three deep in solidly above-average starters. One could argue that none of these guys is a true top-flight pitcher, of course. Still, if Colorado simply adds a decent innings-eater, that would help an already solid, and vastly underrated, rotation.

Really, it’s the lineup that needs improvement, something that gets obscured due to Coors Field’s incredibly high run environment. DJ LeMahieu and his .351 BABIP-inflated batting line don’t inspire confidence at second base. First base has been worse, with Todd Helton a sub-replacement-level player at this stage of his career, Jordan Pacheco a lousy platoon mate, and the holes in Tyler Colvin’s swing possibly being too much for his power to overcome. There are some aggressive options available here, starting with giving destroyer of minor league pitching Corey Dickerson a clean shot at an everyday outfield job and moving Michael Cuddyer to first. But a more likely scenario (assuming the Rockies stay in the race) would involve a trade, maybe something like acquiring Adam LaRoche from the Nats if the rumors that Washington might want to buy and sell at that position at the same time are true.

SAN DIEGO PADRES

In another case of a team that might try to buy and sell at the same time, multiple reports have the Padres seeking starting pitching help … while also potentially fielding offers for Edinson Volquez. The devil, in this case, is in the details. The Padres are said to be after a “front of the rotation guy” while Volquez shoots for his second straight season leading the National League in walks.

If the rotation’s problems stopped with Volquez, the Padres might be able to get by, especially with several promising starting pitching prospects making their way up through the system. But the problems run much deeper, extending to a pitcher with one of the best records in the league. Even after getting torched for six runs on six hits and five walks Tuesday night, a quick glance at Jason Marquis’s 9-3 record might lead you to conclude that he’s been San Diego’s most effective starter. In reality, he’s been the worst pitcher on the staff, leading the league in walks allowed by a wide margin (Volquez is second) and posting a hideous 5.75 FIP that belies his seemingly acceptable 3.99 ERA. In reality, Eric Stults has been the rotation’s only consistently better-than-average starter.

Meanwhile, the lineup’s quality suggests that the Padres contending in a weak division this year might not be that far-fetched. That’s assuming they get some of their top players back from injury soon. There’s good news on that front, with Bud Black saying Tuesday that he expected Jedd Gyroko back by the weekend, Everth Cabrera in about a week, and Yonder Alonso in 10 to 12 days. Combine those expected returns with the emergence of Kyle Blanks and the Padres could have some attractive talent to offer in trade, even if they choose not to dip into their extensive cache of minor league pitching prospects.

SAN FRANCISCO GIANTS

The hot rumor is that they’re after Ricky Nolasco, but given how motivated the Marlins are to move the only player on their roster making more than $3 million this year ($11.5 million and in his walk year, actually), we should probably expect a lot of buzz on that front.

The bigger story is that the Giants’ biggest need would be a starting pitcher. At the start of the season, their rotation featured a two-time Cy Young winner (Tim Lincecum), a right-hander in Year 2 of a $127.5 million deal (Matt Cain), a one-time Cy Young winner who’d dropped to fringe starter status only to pitch lights-out late last season and through the playoffs (Barry Zito), and an all-timer of a reclamation project who’d posted ERAs of 2.71 and 3.37 after five years out of the big leagues (Ryan Vogelsong). All four of those starters have disappointed: Lincecum has shown flashes of excellence, but his overall numbers underscore the mediocre pitcher he’s become; Zito continues to mix solid starts with nightmarish ones; and Vogelsong’s on the DL and not expected back for weeks. The good news is that Cain has rebounded after a horrific start to the season, and Madison Bumgarner has emerged as the new ace of the staff. Still, the Giants could at the very least use a replacement for Vogelsong’s spot, especially now that his replacement, Chad Gaudin, has also hit the DL. And realistically, a team with playoff aspirations should do everything it can to avoid starting the 2013 versions of Lincecum and Zito, if the Giants do in fact make it that far.

Problem is, starting pitching isn’t their only problem. Angel Pagan might be lost for the season, leaving Andres Torres to play far more than he should. Marco Scutaro’s playing through a finger injury that could knock him out of the lineup at any time. That’s before one considers the injury-prone Pablo Sandoval having just returned from a foot strain, with the Giants knowing he hasn’t played more than 117 games since 2010. Meanwhile, the Giants’ bullpen, consistently dominant throughout the team’s recent run of success, is merely competent in 2013, thinner than it’s been in a long time.

The good news is that no one in the NL West looks strong enough to run away with the division. Because as of now, this is one of the weakest rosters the Giants have fielded in years.

LOS ANGELES DODGERS

Normally you’d look at a team eight games under .500, tied for the fourth-worst run differential in the league, and figure the most logical step would be to sell a few veterans, regroup, and try again next year.

Not so for the spend-to-the-moon Dodgers. With their $7 billion TV contract in tow and hyperaggressive owners calling the shots, it’s tough to imagine L.A. rebuilding anytime in the next decade, let alone during a season in which its NL West competition looks so mediocre. The decision not to re-sign Hiroki Kuroda after the 2011 season looks worse and worse every day, to the point where even after last summer’s Josh Beckett trade, the investment in Hyun-Jin Ryu, and the Greinke megadeal, the Dodgers are still trotting out Stephen Fife or Chris Capuano every fifth day. That makes the rumors that L.A. also could be a Nolasco suitor look downright logical. The bullpen needs help too, with Brandon League needing a mysterious three-month DL stint for a tummy ache if he’s to salvage his season.

On the plus side, the lineup’s nearly back to full strength, with Carl Crawford the lone remaining regular still on the mend. Crawford’s return will create an outfield logjam, given Matt Kemp returned Tuesday night, Andre Ethier’s in the first year of a five-year, $85 million contract, and Yasiel Puig is busy eating planets. The Dodgers really, really have to hope Ethier gets hot in the next couple weeks. He’s becoming an intriguing commodity if his power returns and the Dodgers commit to swallowing a big chunk of his salary. Given the young talent L.A.’s lost with last summer’s blockbuster and other moves, strip mining the rest of the system for pitching help or to upgrade over Mark Ellis at second would be a tough blow; getting real value for Ethier could help the Dodgers avoid that fate.