MLB Playoff Watch: Who is each contender's biggest X-factor down the stretch?

It takes a certain level of uncertainty to create an X-factor. It could be a struggling player finding his way, an injured player returning to health, or a spark plug coming out of nowhere to fundamentally alter a game or a series. 

Typically, it's not a superstar, though for this exercise we thought it might also be worth including some underperforming talents whose turnarounds could ultimately define a team's season. 

For each contender, here's one player who could make the difference between a decent year and one that won't be forgotten: 

(Only teams within six games of a playoff spot entering Monday were considered for this ranking.) 

THE ROOKIE RIGHTIES …

19) RHP Taj BradleyTampa Bay Rays (71-49, AL wild card No. 1)

OK, so he's the only player on this list who's not currently with the big-league club. Bradley's rookie year hasn't gone exactly according to plan, but the potential is obvious, and it's hard to imagine he won't play a role in the Rays' stretch run after Shane McClanahan's injury if he can build on the five scoreless innings he threw in his last outing at Triple-A Durham. 

18) RHP Grayson RodriguezBaltimore Orioles (73-45, 1st in AL East)

The Orioles can mash, but they'll need a group effort in the rotation to hold off their formidable division foes. Rodriguez has helped stabilize the group recently in a volatile rookie season, giving hope for what could be ahead by holding opponents to a .536 OPS over his last five starts. 

17) RHP Eury PérezMiami Marlins (62-57, NL wild card No. 3)

The Marlins were 51-39 when they decided to rest their blossoming 20-year-old superstar. The break doesn't appear to have helped either the Marlins, who dropped 16 of their 23 games without him, or Pérez, whose first two starts back have been shakier than usual. Still, he's the type of impact talent who gives Miami a chance to hold onto a playoff spot. 

THE MERCURIAL MASHERS…

16) OF Tommy PhamArizona Diamondbacks (59-59, 2.5 games back of NL wild card)

The D-backs needed more outfield pop, particularly against left-handed pitching. Pham boasts a .850 OPS against lefties on the year, though he is still looking to recapture his first-half form (.836 OPS) in the second half (.677 OPS). Perhaps, his first home run with his new club Friday gets him — and the sliding D-backs — going again.  

15) OF Giancarlo StantonNew York Yankees (60-58, 5 games back of AL wild card)

There may be no saving this Yankees offense, which is still getting very little beyond reigning MVP Aaron Judge, but it certainly can't afford Stanton to be a league-average hitter when healthy. Stanton has demonstrated more power since the break, but the Yankees need much more of it to make any sort of move. 

14) OF George SpringerToronto Blue Jays (66-54, AL wild card No. 3)

Over the past 30 days, the Toronto offense has a sub-.700 OPS against right-handed pitchers. Springer has a .501 OPS against righties in that timeframe after an abysmal month of July. But in the midst of his worst offensive season, the four-time All-Star looks to be heating up in August. 

THE RETURNING HURLERS… 

13) RHP Hunter GreeneCincinnati Reds (62-58, 0.5 games back of NL wild card)

The Reds need a rotation boost, and it didn't come at the deadline, so the health of their most talented young arm will be crucial. Greene could return from his hip injury before the end of the month. 

12) LHP Max FriedAtlanta Braves (75-41, 1st in NL East)

The Braves are cruising to a division title with one of the most complete teams in baseball, but their rotation has looked shaky over the past month. The return of Fried should stabilize the group. The most important thing is his October availability. 

11) RHP Brandon WoodruffMilwaukee Brewers (65-54, 1st in NL Central)

Freddy Peralta and Corbin Burnes (until his most recent start) have looked great, but this rotation is completely different with Woodruff back in the mix. He has a 1.99 ERA in his four starts this year and could make the difference in holding off the Cubs and Reds. 

10) LHP Chris SaleBoston Red Sox (62-56, 3.0 games back of AL Wild Card)

Sale struck out seven of the 16 batters he faced in his first start back in more than two months. Sure, it was against the Tigers, but that's a welcomed sight for a Red Sox team with a bottom third rotation ERA in baseball. 

9) RHP Nathan EovaldiTexas Rangers (70-48, 1st in AL West)

Eovaldi looked like a potential Cy Young candidate before his forearm strain, which has held him out for nearly a month. It's unclear exactly when he'll return, and Max Scherzer and Jordan Montgomery could provide the Rangers enough depth in the rotation to hold off the Astros until that time, but Eovaldi's presence for a deep October seems crucial. He has the lowest ERA (2.69) among all qualified AL starters. 

THE POTENTIAL DIFFERENCE MAKERS… 

8) RHP Jameson TaillonChicago Cubs (61-57, 0.5 GB of NL wild card)

While Cody Bellinger led an offensive fusillade to spawn Chicago's surge, Taillon's resurgence helped mask the flaws of a rotation dealing with some serious second-half struggles. Taillon looked to have turned a corner with a 2.17 ERA over his past six starts — all wins — until Sunday, when he was roughed up for eight runs in three innings.

7) RHP Alex Cobb —  San Francisco Giants (63-55, NL wild card No. 2) 

From the start of May through July 15, the Giants won 10 of the 11 games that Cobb started. Since then, the 35-year-old All-Star has allowed at least five runs in three of his past four starts. Cobb will need to find his previous form for the Giants, who are regularly using only two traditional starters in their rotation, to snap out of their second-half funk. 

6) SS Jeremy PeñaHouston Astros (68-51, AL wild card No. 2) 

Last postseason looked like Peña's emergence, but the Gold Glover and World Series MVP has been unable to build on that offensive display. Things have started looking up in August, and if he gets hot again, good luck stopping the reigning champs. 

5) LHP Julio UríasLos Angeles Dodgers (71-46, 1st in NL West)

With the Dodgers striking out on adding an ace at the deadline — though all their additions have been tremendously productive — it's essential that Urías finds his ace form for the club to make a deep October run. His performance in August (3-0, 1.50 ERA, 0.722 WHIP), including Sunday's 12-strikeout effort, brings optimism for what's ahead. 

THAT SHORTSTOP CLASS …

4) SS Xander BogaertsSan Diego Padres (56-52, 5.5 games back of NL wild card)

3) SS Carlos CorreaMinnesota Twins (62-58, 1st in AL East) 

2) SS Trea TurnerPhiladelphia Phillies (65-54, NL wild card No.1) 

Who would've guessed the highest OPS from the offseason's stacked free-agent shortstop class would belong to Dansby Swanson?

The three guys listed above are typically too good to make an X-factor list, but it's been a forgettable year for the trio of $200-plus million shortstops. Turner and Correa are both hitting below league average on the year, though they look to be turning a corner. If Turner finds his usual form — his past 10 games represent his best stretch of the season — a return trip to the World Series suddenly seems much more realistic for Philadelphia. 

THE GAME-CHANGER…

1) CF Julio RodríguezSeattle Mariners (63-54, 1.5 games back of AL wild card)

The Mariners didn't do much to address their lackluster offense at the deadline, so they need a superstar performance the rest of the way from Rodríguez to stay in the mix. So far, the reigning AL Rookie of the Year is delivering. After posting a sub-part .721 OPS in the first half, he's been impossible to ignore since — and so has Seattle.

Rowan Kavner covers the Dodgers and MLB as a whole for FOX Sports. He previously was the Dodgers' editor of digital and print publications. Follow him on Twitter at @RowanKavner.