Baseball is a team game -- and an unpredictable one, at that. So the idea that a single player can swing a team's fortune one way or another can be faulty. Last year, the Angels were a game under .500 (19-20) in the six weeks Michael Trout was on the disabled list and a game under .500 while he was on the active roster (61-62). The Dodgers are an incredible 74-25 over the past two seasons when Clayton Kershaw has been on the shelf with back issues.
Everything written below, then, is not to be treated as gospel (and if it were treated as gospel, that would be an unusual religion).
Still, there's no denying that there are certain players whose long-term absence or major performance decline would have their clubs scrambling, if not stumbling. With a tip of the cap to the New York Post's Ken Davidoff, whose recent column on Noah Syndergaard as the Mets' linchpin inspired this piece, here are my picks for the most indispensable player on each team for 2018.
A's:Matt Olson
You'd have to imagine any 2018 scenario that involves the A's unexpectedly reaching October would have to involve Olson picking up where he left off at the end of '17 (1.003 OPS in 59 games).
Angels: Mike Trout
Though I cited the Halos' 2017 record with and without Trout above, I'm not dumb enough to go with anybody else here. Last year's thumb injury thankfully cost Trout just six weeks. If Trout were to miss, say, half of '18, you could probably kiss that hope of making a significant rise in the standings goodbye.
Astros:Jose Altuve
The Astros have so many great pitchers and position players that they force you to make an impossible choice here. But the bottom line is that when you've got a guy getting on base 40 percent of the time, as Altuve has the past two years, that -- to say nothing of his energy and pure joy -- is pretty much impossible to replicate should he go down.

Blue Jays:Josh Donaldson
Donaldson's right calf injury last spring compromised his first half and, in turn, compromised the Blue Jays' playoff hopes (they went 41-47 before the All-Star break).
Braves:Freddie Freeman
Ronald Acuna Jr. could supplant him as the Braves' best player before long, but the club's only hope of sneaking into the National League Wild Card picture ahead of schedule involves Freeman, who has a .908 OPS and a 147 OPS+ over the past five years, doing his thing.
Brewers:Chase Anderson
Maybe it's Jimmy Nelson, the staff ace expected to return from shoulder surgery midseason. But if we limit ourselves to active members of Milwaukee's roster, then Anderson's importance after a breakthrough year (2.74 ERA, 160 ERA+) is temporarily augmented by Nelson's absence and the Brewers' still-unmet need for a rotation boost.
Cardinals:Marcell Ozuna
Love Tommy Pham, but last year was evidence of the Cardinals' screaming need for a thumper, and this is the guy they reeled in. If Ozuna repeats his .924 OPS from last season, the Cards are scary. If he repeats his .691 OPS of 2015, they're a different sort of scary. And if Ozuna gets hurt, it's back to square one.
Cubs:Kristopher Bryant
In just 457 big league games, Bryant has already amassed 21.6 FanGraphs-calculated Wins Above Replacement and played every position other than pitcher, catcher and second base.

D-backs:Paul Goldschmidt
A top-three finisher in the NL Most Valuable Player Award voting in three of the past five seasons, Goldschmidt's stats have been static (in a good way) for Arizona in all its various iterations (and uniform combos).
Dodgers: Clayton Kershaw
As with Trout, we're thinking big picture here, rather than getting wrapped up in past samples. Yes, the Dodgers, who are extraordinarily deep, played extraordinarily well without Kershaw in 2016 and '17. But he's the only guy on their staff counted on for a traditional starter's workload, and if he were to blow out his elbow tomorrow, it would expose their rotation mixing and matching to a much greater degree of difficulty.

Giants:Buster Posey
It's him or Madison Bumgarner, whose dirt bike accident was a fitting piece of the wreck that was 2017. Posey gets the nod because he plays every day and still projects to be the Giants' most valuable position player after the additions of Evan Longoria and Andrew McCutchen.
Indians:Andrew Miller
Losing Corey Kluber or Francisco Lindor would be crushing, though the Indians do have rotation and lineup depth. A Tribe bullpen that already leaned heavily on Miller (he pitched 18.2 percent of Cleveland's postseason innings over the past two years) has been thinned a bit by free agency, making the lefty's high-leverage versatility perhaps more important than ever.
Mariners: Felix Hernandez
I might have gone in a different direction at this stage in King Felix's career, but I'll defer to general manager Jerry Dipoto, who said: "If Felix can give us the 25 or more starts he gave us in 2016, we are going to be a good team. If Felix gives us 16 or less, like was the case last year, we are going to have to answer a lot of questions."
Marlins: Lewis Brinson
When what you're selling is hope and faith in the big picture, your No. 1 prospect playing every day in center field is a pretty darn important piece, raw or not.
Mets: Noah Syndergaard
Hard to disagree with Davidoff's point that on a Mets club absolutely reliant on the health and potential of the starting staff, another long-term absence for Thor would be a particularly big bummer. Steamer gives Syndergaard the third-highest WAR projection (5.5) of any pitcher, trailing only Kershaw (5.8) and Kluber (5.7).

Nationals:Bryce Harper
Last year, Harper hyperextended his left knee in mid-August and missed six weeks. Before his injury, the Nats scored 5.4 runs per game with an .811 team OPS. With Harper gone, they scored 4.1 runs per game with a .668 OPS. Sometimes, this game isn't as complicated as we make it out to be.
Orioles:Manny Machado
Yes, the O's might dispense of this indispensable talent this year, and that's the point. If Machado were to sustain a significant injury, it would hurt not only Baltimore's competitive chances, but its chances of obtaining real value for him in the trade market.
Padres:Eric Hosmer
Didn't take him long to assume this spot, did it? Hosmer's now the example and the sounding board for the Padres' young core to look up to and turn to.

Phillies:Aaron Nola
Jake Arrieta's arrival takes some pressure off Nola, but his importance for the short- and long-term is still pretty clear. While Nola had a 3.54 ERA in 27 starts last year, Vince Velasquez, Nick Pivetta, Jerad Eickhoff, Zach Eflin and Ben Lively combined for a 5.24 ERA in 91 starts. Nola's health and performance will be vital as the Phillies begin to graduate from rebuild mode to contention mode.
Pirates:Starling Marte
Whatever air was in the Pirates' season in mid-April last year was promptly let out when Marte received an 80-game suspension for a positive PED test. His importance is underscored all the more by McCutchen's absence.
Rangers:Cole Hamels
The Rangers' rotation beyond Hamels ranges from the iffy (Doug Fister, Matt Moore, Mike Minor, Matt Bush) to the injured (Martin Perez).
Rays:Chris Archer
If the Rays' ace was important when the club had a five-man rotation, I guess you'd have to say Archer is at least 5 percent more important now that they're going with a four-man rotation.

Red Sox:Chris Sale
Sale is the only non-if in Boston's rotation these days. Even in a 2017 in which David Price made just 11 starts and Rick Porcello went backward, the Red Sox became a more pitching-oriented team in large measure because of Sale's success.
Reds:Joey Votto
You know about the production, but, as Votto has matured into a better teammate in recent years, he's become even more valuable.
Rockies:Nolan Arenado
Arenado was the only member of the Rox with a road weighted-runs-created-plus mark (wRC+) that rated as significantly better than MLB average last season. You combine that with the glove and intensity he brings to the table.
Royals:Salvador Perez
Perez is the heart and soul of the club, all the more now that Hosmer is gone. These could be trying times for a team that recently achieved greatness, and so his production, work with the pitching staff and positivity are vital.
Tigers:Michael Fulmer
Every Tigers starting pitcher not named Fulmer or Justin Verlander last year had an adjusted ERA+ at least 13 percent worse than MLB average. And Verlander is long gone.
Twins:James Dozier
Until or unless Miguel Sano and Byron Buxton put it all together for a full season, Dozier's importance to this lineup can't be overstated. Over the past two seasons, he has been responsible for 13.8 percent of their runs created.
White Sox: Yoan Moncada
Hate to put too much pressure on the kid entering his first full season in the big leagues, but if Moncada doesn't begin to break out as anticipated, the Sox's rebuild plan looks a lot different.
Yankees:Luis Severino
The arrival of Giancarlo Stanton and the assumed health of Gary Sanchez and Greg Bird give the Yanks cushion should Aaron Judge regress in his sophomore year. But injury or regression for Severino would rob them of their greatest source of starting upside and likely propel them to a more aggressive tact in the trade market.