How do you keep rolling when it appears the best free agent slugger left on the market — 37-homer Josh Donaldson — doesn’t want your $100 million offer and prefers to stay in Atlanta for a few dollars more?

The Washington Nationals’ answer last season was to create “old platoons” — an out-of-vogue notion — at catcher, first base and second base. Almost unnoticed, the Nats got 324 RBI from those platoons, more than any team in the majors got at those three positions. With 95 homers, too.

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Now, the Nats are at it again, building four platoons, or positional rotation spots, by adding third base to the list in Rendon’s absence.

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What is the Nats’ hope? If the infielders they have assembled thus far (plus Ryan Zimmerman soon) play as they did last year, the Nats have four positions — half their everyday lineup — that can average 100 RBI. They’ll do it by collecting modest-cost veterans at every spot.

If the Nats, who also have dramatically improved their bullpen since this past summer, manage to equal, or surpass, their 93 wins of last season, it will be a masterpiece of make-do team reconstruction.

Next October? Who knows? But we know what this offseason has brought so far. One key in the adaptable Nats’ ability to stay near the top is finding players — this time Will Harris, Starlin Castro and Eric Thames, plus re-signing Asdrúbal Cabrera and Daniel Hudson — who underwhelm, then overperform.

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Similar players, such as Howie Kendrick, Kurt Suzuki and Yan Gomes, are still around. Others, such as Brian Dozier, Matt Adams and Gerardo Parra, are gone but will still get rings.

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Many in the majors see the current Nats infield — with Castro, Cabrera, Thames, Kendrick and, let’s assume, a low-cost Zimmerman, too — and assume they still want Donaldson. “How does this muddled assemblage fit together,” they wonder, “and will they be comfortable in their new jobs?”

What new jobs?

If these infielders played the same number of innings at exactly the same positions they played last year, the Nats would have almost exactly the proper number of innings allocated to each position. This isn’t a hodgepodge. It is a great fit.

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In 162 games, you need about 1,458 man-innings per position.

Last season, Castro, Cabrera and Kendrick played 1,012⅔ , 248 and 165⅔ innings at second base. That’s 1,426⅓ innings. Close enough to 1,458? You don’t think this is just an accident, do you?

Last season, Cabrera, Castro, Kendrick and utility man Wilmer Difo played 812, 366⅔ , 99⅓ and 35⅔ innings at third base. That’s 1,313⅔ innings. Once again, a nice fit and easy to adjust.

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Last season, Thames, Zimmerman and Kendrick played 782, 346 and 336 innings at first base for a total of 1,464. Almost 1,458 on the nose.

Nothing is this simple. Players get injured, such as Trea Turner for 39 games in 2019. Pitching matchups dictate lineups. But the offseason can be played on one surface: paper. The Nats have executed it.

Sharp fans will say Kendrick may not hit as well as he did in 2019 or that Cabrera will not have such a high RBI total (91, counting his time with Texas) again. Maybe.

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But what if Castro hits just remotely as he did in his last 78 games with Miami, slashing .314/.344/.571 with 17 homers and 53 RBI?

In Castro, has General Manager Mike Rizzo found another late-career launch-angle convert, in the mold of Daniel Murphy, Kendrick and Suzuki? Last season, Castro’s groundball-to-flyball ratio improved by 27 percent.

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Let’s assume the good and the bad is a wash. What if the Nats in those first, second and third base platoons duplicate their 2019 stats?

Including Zimmerman, the current Nats at first, second and third base would have 2,154 plate appearances — a normal full-season total for three positions — with 88 homers and 307 RBI. Yes, an average of 29 homers and 102 RBI per position.

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No, this is not as good as Rendon last season. But if Turner stays healthy, some of the gap might be made up.

The Nats’ other important roster moves have been adding Harris (2.36 ERA over the past five years) and bringing back Hudson (1.44 ERA after his August arrival). How much can they improve the worst bullpen by ERA (5.66) of any team that made the postseason since 1903?

First, we must grasp what a nasty fluke of bad luck on top of bad construction befell the Nats and Rizzo last year. Every pitcher used in the eighth inning fell into the same abyss. Just as batting orders are synergistic, so are bullpens, with both bad and good reinforcing themselves.

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That cascade collapse resulted in a bizarre cast — Kyle McGowin, Dan Jennings, James Bourque, Michael Blazek, Austen Williams — joining Trevor Rosenthal and Kyle Barraclough to build a four-month bullpen pyre. The worst Nats relievers, none still around, allowed 89 earned runs in 66 innings — a 12.14 ERA!

Now Harris will wrestle with the cursed eighth inning. It’s bad science to substitute his mere 10 earned runs in 60 innings for the arsonists’ stats. But you can whistle as you walk through the snow and note that 89 sure seems like a much bigger number than 10. And you haven’t factored in Hudson’s full-season value yet.

Every World Series winner faces a complex winter, fighting complacency and, often, the leaping market value of its players, whether they get more (Stephen Strasburg, $245 million) or leave for vast treasure.

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Nonetheless, the two seasons after winning a World Series should never be wasted. The chances of playing deep into October are always low. But they are best after you just won it all — because you are exceptional, and pressure-tested, too.

Since the wild card arrived in 1995, major league champs have averaged 95.6 wins in their title year, then 87.8 and 88.5 their next two years. Few stay tiptop year after year, but most remain contenders. These 24 champs since 1995 have gotten back to the playoffs 27 times within the next two years (or 57 percent of their opportunities) with 19 trips to the league championship series (40 percent) and 12 return trips to the World Series (26 percent).

You must take a shot at those two-year windows. Few of those teams suffered a subtraction as big as Rendon. That’s why the Nats are now the seventh pick to win the 2020 World Series. But their greatest strength, their Big Three aces, is locked down through 2021.

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Often, such prizes are sought the way the New York Yankees are trying to do it: signing Gerrit Cole from their toughest AL rival for $324 million. It’s considerably more challenging, and to some, even more appealing, to respond to high hurdles — such as losing Harper and Rendon, and, it appears, Donaldson, too — by turning to sensibly priced veterans such as Harris, Castro, Hudson, Thames and Cabrera.