Both Jason Hammel and John Lackey have been important pieces for Chicago's spectacular season, but manager Joe Maddon will likely only have room for one of them when October rolls around.

The Cubs built the best team in baseball over the past five years by focusing first and foremost on position players. Whenever anyone talked about the franchise’s rebuilding plan, the conversation always began with the foundation of quality hitters, like Kris Bryant, Anthony Rizzo and Addison Russell.

And yet, Chicago's pitching staff is just as responsible for the team’s incredible success this season. Cubs starters lead the majors with a 2.92 ERA, more than a half-run better than the Mets' 3.53 mark that is second best. Ace Jake Arrieta hasn’t been as consistently dominant as he was en route to winning the NL Cy Young Award last season, but he has still notched 16 wins, a 2.84 ERA, a 1.05 WHIP and 168 strikeouts. No. 2 starter Jon Lester is fourth in the majors in ERA (2.61), ninth in WHIP (1.04) and has 15 wins. Kyle Hendricks, who opened the season as the fifth starter in the rotation, leads the majors with a 2.07 ERA, is one of two pitchers with a WHIP south of 1.00 (the other is Washington's Max Scherzer) and is a legitimate Cy Young candidate.

After those three, however, manager Joe Maddon, pitching coach Chris Bosio and top executives Theo Epstein and Jed Hoyer will have to decide who takes the ball when the Cubs need a fourth starter in October. Two veteran righthanders, Jason Hammel and John Lackey, have both had moments this season when they looked like the team’s fourth-best starter, but only one can get the nod. As Chicago continues to roll toward the NL Central crown, it's worth considering one of the few pressing questions the club must face before the playoffs start: Who should start a Game 4?

Brian Rothmuller/Icon Sportswire (Hammel); Matt Slocum/AP (Lackey)

The case for: Jason Hammel

Hammel is enjoying the best season of his career, pitching to a 3.50 ERA and a 1.19 WHIP while racking up 127 strikeouts in 151 2/3 innings. His FIP and xFIP sit at 4.38 and 4.37, respectively, and he hasn't missed a start all year. Thanks in part to Chicago's potent lineup, he already has a career-high 14 wins, and he’ll likely make three more starts in the regular season.

Unlike the two previous years, Hammel has not followed impressive first-half performances with a second-half swoon. He was so bad in 2014 after the Cubs dealt him to Oakland in the trade that brought back Addison Russell that the A’s didn’t even make an effort to re-sign him, letting him instead go right back to the Cubs as a free agent. In 2015, he followed a 2.86 ERA before the All-Star break with a 5.10 mark afterward. This year, however, Hammel actually got better as the season went on; he had a 3.46 ERA in the first half and a 2.54 mark in his first nine starts of the second half before being tagged for nine runs (eight earned) in his most recent outing, on Tuesday against the Brewers.

Against potential postseason foes, Hammel has not faced the Giants, but he got rocked by the Mets (surrendering 10 runs in four innings on July 1) and stung by the Dodgers (he didn't make it out of the third inning against them on Aug. 27). He was decent against the Nationals (allowing four runs in 12 innings over two starts) but has been most effective against St. Louis, going 2-1 with a 2.84 ERA, a 1.00 WHIP and 13 strikeouts against four walks in 19 innings spread over three starts. He’ll get another turn against the Cardinals next week.

There is a potentially hidden factor that could work in Hammel’s favor: Maddon is not shy about having a quick hook in the playoffs. In five of Chicago’s nine playoff games last season, Maddon lifted his starter before the sixth inning. Four of those times, he had his starter out before the end of the fifth. It’s possible Maddon and Bosio would have more faith in Lackey as a reliever-on-call than Hammel.

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The Case For: John Lackey

Lackey's experience undoubtedly gives him an edge over Hammel. The 37-year-old has made 23 appearances (including 20 starts) and thrown 127 1/3 innings in his postseason career. He’s 8-5 with a 3.11 ERA and 1.22 WHIP in those games, and he won World Series clinchers for the Angels in 2002 and the Red Sox in 2013. Hammel, on the other hand, has made five starts in the postseason, amassing a 6.98 ERA and a 1.76 WHIP in 19 1/3 innings. Lackey has been there a lot and succeeded. Hammel has been there a few times and struggled. That can't be overlooked.

Save for a hiccup in the middle of the season, Lackey has given the Cubs all they could have expected from him after he signed a two-year, $32 million contract with them last off-season. He has a 3.36 ERA, a 1.02 WHIP and 160 strikeouts in 163 1/3 innings. He has toed the rubber 25 times this season, notching a quality start in 18 of those trips. He returned last week from disabled list stint caused by a shoulder strain and allowed one earned run on one hit in five innings in an eventual Cubs win over the Giants.

Lackey has made nine combined starts this season against Chicago's most likely postseason opponents, the Cardinals, Nationals, Mets and Giants this season (he has not pitched against the Dodgers). In those games, he has a 2.29 ERA, a 0.92 WHIP and 58 strikeouts in 51 innings. In other words, Lackey has been even better against potential playoff foes than he has against everyone else.

Finally, there shouldn’t be any concern about Lackey fading in the postseason. He is on track to throw 180 innings for the fourth straight season. He has thrown at least 200 innings six times in his career, including as recently as last season, and he returned without incident from his recent DL stay.

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The Verdict

After breaking it down, it seems clear what the Cubs' decision will be. Lackey has a major edge in postseason experience over Hammel, and he has been better this season both overall and against their possible postseason foes. Hammel, meanwhile, has been knocked around in two of his last four starts, bringing back memories of his second-half issues the last two years. Hammel started two games in the postseason in 2015, and he didn’t go longer than three innings in either one. The Cubs had to lean on him then, despite his problems after the All-Star break. Then they signed Lackey for additional rotation depth and that is no longer the case. Expect Lackey to get the first chance in a potential Game 4.