The Mets and Cubs will play their most important three games this week since their four-game faceoff in October 2015.

That the Mets’ strength has remained the same — with basically the same cast — is stunning for the major leagues in this era of pitching injury and transience. The Mets swept the Cubs in the 2015 NLCS for many reasons — you might, for example, remember Daniel Murphy became Ted Williams. Nothing was more instrumental, though, to the Mets reaching their first World Series since 2000 than their young, overpowering rotation.

Matt Harvey, Jacob deGrom, Noah Syndergaard and Steven Matz combined to hold the Cubs to six runs in 24 1/3 innings, striking out 29. Sandy Alderson’s grand design to build around power pitching peaked that fall — even with Zack Wheeler in the midst of missing the first of two seasons after Tommy John surgery.

But all the problems of building around a rotation manifested in the following years, namely with the fragility of the position: Wheeler was slow to heal, Harvey fell apart and Syndergaard battled the strains and sprains associated with heaving a ball with such force.

Yet, Alderson’s Mets GM successor saw the same possibilities. Brodie Van Wagenen doubled down on the rotation. He didn’t trade deGrom despite an industry salivating for the ace. Instead, Van Wagenen extended deGrom two years before he reached free agency. He dangled Syndergaard in the marketplace last offseason and in July, yet retained him. Van Wagenen not only kept Wheeler with free agency due after this season, but traded two of his better prospects from a limited stock to add Marcus Stroman.

The reward is the Mets entered the three-game set at Citi Field two games behind the Cubs for the second wild card. They surged into contention by going a major league-best 27-13 to open the second half. And the main reason has been as big a mystery as what day follows Tuesday.

The Mets rotation led the majors in second-half ERA (2.71) and OPS against (.626). The Mets were the only team averaging more than six innings per start (6.06) after the All-Star break, allowing them to better protect and deploy a dubious bullpen. In a season when the ball is flying, Mets starters had permitted an MLB-low 19 second-half homers (Yankees starters had yielded the most: 58). Among pitchers with at least seven second-half starts, deGrom (1.04) was second in the majors in ERA, Syndergaard fourth (1.82) and Matz 17th (2.64). The only other team to have three starters in the top 20 was the Dodgers (Clayton Kershaw, Walker Buehler, Hyun-jin Ryu).

That those three Mets also started in the 2015 NLCS for the Mets defies the trends of the modern game. DeGrom won NL Rookie of the Year in 2014, and Syndergaard and Matz both debuted in 2015, Matz with six late-regular-season starts. The only other team that had three starters make at least five starts in 2015 and at least five this year is the Indians, and Trevor Bauer was traded in July to the Reds while Carlos Carrasco and Corey Kluber have spent the majority of this season on the IL.

This is the second straight year the main Mets foursome has remained mostly healthy. The team failed to capitalize on deGrom, Matz, Syndergaard and Wheeler each making at least 25 starts last year. They each had at least 24 starts (and counting) this year with far greater team success. A playoff appearance would probably motivate a continued commitment to the group.

The only sure return is deGrom, who will enter the second season of his five-year extension in 2020. The Mets will have to decide whether to make the qualifying offer (about $18 million) or try to extend Wheeler. They will be tempted again to market Syndergaard (a free agent after 2021), perhaps Stroman (2020) and maybe Matz (2021). But having used two of their close-to-the-majors rotation prospects, Justin Dunn and Anthony Kay, in trades since the end of last year, the Mets do not have obvious plug-ins, unless they were to get them in, for example, a Syndergaard trade.

For now, Syndergaard and deGrom will start in Games 2 and 3 of this series against the Cubs as surely as they did Games 2 and 3 in the 2015 NLCS. The strength then, the strength now. A pitching case of deja threw.

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