The Nationals’ pitching staff may add a high-leverage reliever during the season. (Jonathan Newton/The Washington Post)
Sports columnist

The Washington Nationals will have 25 players active for Monday’s opener against the Miami Marlins, and much consideration has gone into selecting each one, down to the final outfielder and the last reliever. Stephen Strasburg will take the mound, Bryce Harper will play right field, and off they’ll go, pointed toward what they hope will be a fourth division title in six seasons.

But the chances of those same 25 men being the same 25 who would open a playoff series are basically nonexistent. Last year, in taking the National League East, the Nationals used 43 players, 33 of whom appeared in at least 20 games (for position players) or threw at least 20 innings (for pitchers). That’s a lot of recurring characters. Seven players who lined up for introductions with the Nats on Opening Day had been replaced when the team was introduced for a division series against the Dodgers.

So the lesson: Don’t get too used to each and every member of this crew. Prepare for some measure of change.

Every year Mike Rizzo has served as the Nationals’ general manager, he has made a midseason trade. They have been to rebuild (closer Matt Capps to Minnesota for a young catcher named Wilson Ramos in 2010) and to flat-out win (Felipe Rivero and Taylor Hearn to Pittsburgh for closer Mark Melancon last summer). There have been minor blips (Scott Hairston from the Cubs for minor leaguer Ivan Pinyero in 2013) and franchise-rocking errors (Jonathan Papelbon from the Phillies for minor leaguer Nick Pivetta).

Rizzo’s philosophy: win the division in the offseason. Win the World Series at the trade deadline.

This might be a convenient time to point out that the Nats’ record in the former is quite good. The latter: not so much.

This year’s Washington team heads into the season with one glaring question — closer — and several less-obvious ones, not least of which is Strasburg’s ability to make 30-plus starts, but which also includes a completely overhauled defense up the middle, Ryan Zimmerman’s ability to bounce back from the worst season of his career, Matt Wieters’s value as Ramos’s replacement, and on and on.

It may be possible that the Nationals, with questions both major and minor, won’t need even a bit of a tweak. But don’t count on it.

Start with closer, because both how the roster looks now and what the Nats have done in the past are instructive. This offseason, Washington made a run to retain Melancon — who would have been a perfect fit financially, on the roster and in the clubhouse — before he signed with San Francisco. They regrouped to get in late on Kenley Jansen — younger, more powerful and more expensive — before Jansen re-signed with the Dodgers.

When the Nats took their shots at those two, they already had Blake Treinen, Shawn Kelley and young Koda Glover on the team. They added Joe Nathan — who not only is 42, but whose arm is essentially held together by chewing gum and wire — for what seemed like a far-fetched tryout for a roster spot. He was cut on Monday.

What does that tell you? It tells you the Nationals believed the group they had over the winter — the group that, at the back end of the bullpen at least — wasn’t good enough.

How might that serve as a predictor for what’s to come in the summer, when July 31 serves as the nonwaiver trade deadline? Look at Nationals history — and Rizzo’s history — to figure it out.

This isn’t just about Melancon in 2016. It’s also about the trade for Papelbon in 2015. On the day the Nationals pulled off that deal, incumbent closer Drew Storen had saved 29 of 31 opportunities and had a 1.73 ERA. Rizzo publicly presented it as strengthening a strength. Privately, he didn’t trust Storen in the postseason and never intended to enter October with Storen pitching the ninth.

The equation, though, for what a closer is worth in a trade is a difficult one to figure. Last year, before they turned to the Pirates and Melancon, the Nats were inquiring with the Yankees about Aroldis Chapman (who ended up with the Cubs) and Andrew Miller (who ended up with the Indians). There is belief within the Nats’ front office that the package Washington sent to the White Sox this winter for outfielder Adam Eaton — pitchers Lucas Giolito, Reynaldo Lopez and Dane Dunning — would have been enough to land Miller, who became the most dangerous weapon in the postseason while leading Cleveland to the American League pennant.

“But for 70 innings?” one Nats exec said.

Miller, though, would look pretty nice in a Nats uniform right now, transforming that question mark into what would appear to be an obvious strength.

Which leads us back down the same road: Miller’s not here. Neither is another established closer in his prime. Thus, a trade would appear likely.

It’s possible — possible, not probable — that one of these current candidates simply seizes the role and we never revisit it. Even if, say, Treinen shows his hard sinker is now accompanied by a hardened mentality, would the Nats — who still are looking for their first victory in a playoff series — head into October with someone who has never performed the job in that month before?

Welcome those 25 men who are introduced Monday at Nationals Park, because each earned a roster spot. Enjoy the game that plays out after the players leave the first base line and head back to the dugout. But remember: there are 161 more games — and who knows how many more players — that will follow.