Matt Wieters, in a down offensive season, has made an impact on the Nationals’ rotation. (Brad Mills/USA Today Sports)

When the Nationals signed Matt Wieters a week into spring training, a reasonable question arose: Why? Why did they commit $10.5 million to an unknown commodity when a guy they drafted and former all-star, Derek Norris, looked ready for a bounce-back season?

General Manager Mike Rizzo seemed to be thinking similarly for much of the offseason, not willing to dish out the kind of deal Wieters seemed likely to command for a player he did not always consider a major upgrade. Then he flew to Atlanta to meet with Wieters and was, as he put it, blown away. Wieters had a plan for every one of his starters, an approach catered to Gio Gonzalez that might just revive him and a clear commitment to out-planning the opposition whenever possible. Rizzo liked what he heard. The Nationals pulled the trigger. Out went Norris.

Norris has encountered off-field trouble since the Nationals let him go, so any comparison between what the Nationals might have had and what they’ve gotten is not useful here. But Wieters has not produced according to his resume. He is in the midst of the worst offensive season of his career. He has allowed more stolen bases than any catcher in the big leagues. He has also made what is — anecdotally and statistically — a substantial impact on a pitching staff that was pretty good to begin with, an impact most felt in the way Nationals starters are pitching hitters.

Determining who is responsible for what pitch someone throws when can be difficult. General consensus and baseball code state that the pitcher is responsible for the pitch he throws.

“The guy on the bump, it’s his game. He’s ultimately responsible for what he throws,” Nationals pitching coach Mike Maddux said. “But leadership behind the plate helps take decision-making pressure off the pitcher.”

Catchers see things hitters don’t, know hitters in ways pitchers can’t and read swings from a more telling angle that can inform them in crucial situations. But catchers take their orders from somewhere, too. They are given a scouting report and game plan by their pitching coach. Nationals starters almost always credit Maddux’s pregame report after a good outing.

“It really is a joint effort. I think that’s what works so well is being able to go over a plan with a pitching coach. He initiates the plan,” Wieters said. “Then it’s also being able to make in-game adjustments. I think the in-game adjustments fall more on the catcher, trying to read what the hitter is doing more than anything else, as well as — not save, but keeping the scouting report fresh so you might be able to use it in a bigger situation than other times.”

The four starters who pitched regularly in 2016, as well as 2017 — Max Scherzer, Stephen Strasburg, Gio Gonzalez and Tanner Roark — are throwing noticeably fewer fastballs than they did last season. They also are throwing a greater percentage of curveballs and change-ups than they did in 2016. Maddux was game-planning for those four in 2016, too. They are the constants. Wieters is the variable and appears to have made a quantifiable effect.

Those four starters threw 59.2 percent fastballs during the 2016 season. If that had been the number for the entire Nationals rotation — and importantly for this analysis, it wasn’t — that would have been the eighth-highest percentage among major league rotations. Those four starters threw combined 12.1 percent curveballs in 2016 (also importantly, all four of them have curveballs), which would have been 11th most among big league rotations. They threw 11 percent change-ups, which would have been 14th most.

With less than a week remaining in the regular season, those same four starters are throwing 53.9 percent fastballs, which is 21st most among major league rotations. They are throwing 15.5 percent curveballs, sixth most in baseball. They are throwing 14 percent change-ups, also sixth most, also a jump from 2016. Wieters seems to be the impetus behind the changing ratios.

“The way the game is kind of going now, everybody wants to hit fastballs,” Wieters said. “It used to be that breaking balls were the power pitches because guys would hit hangers out of the park. Now it’s kind of turning where hitters are wanting to hit the fastball for power and home runs. You’re seeing a lot of backwards pitching where you’re seeing a lot of off-speed in fastball counts.”

“Hitters are now, the bat speed has caught up to the 97, 98 mile per hour fastball,” he continued. “As a catcher, it’s your job to never let that happen. You have to make the hitter feel like he has to respect the curveball or the change-up in any count, and probably more than anything, with the velocity most of this staff has with the fastball, we might be throwing more off-speed to keep that velocity feeling different than it actually is.”

Every one of those big four starters is throwing noticeably fewer fastballs this season than they did last year. Scherzer, who was throwing nearly 60 percent fastballs two seasons ago, is now throwing fewer than 50 percent fastballs. Strasburg is throwing 6 percent fewer fastballs. Gonzalez, on whom Wieters seems to have had the biggest impact, is throwing 8 percent fewer fastballs.

When Rizzo met with Wieters, the veteran’s plan for Gonzalez, who has battled consistency for the last half decade, stuck out to him. Rizzo was hopeful that Wieters’s new approach might spark a revival in the former Cy Young candidate. Gonzalez is fourth in the majors in starter’s ERA, 2.68. Gonzalez has thrown at least 15 starts with six catchers in his career. His ERA with Wieters (2.77) is his lowest with any of them by half a run.

The ERA of the Nationals rotation is currently 3.59. At the end of last season, it was 3.60. If one removes Jeremy Guthrie’s stat-skewing outing in April, the rotation’s ERA is 3.50, though Wieters was behind the plate for that game, too. The Nationals’ starting WHIP is currently 1.19. At the end of 2016, it was 1.19. Their OPS against is currently .688. Last season, it was .679. By those measures, Wieters’s impact on the rotation has been insignificant.

Anecdotally, the starters feel that impact, particularly in Wieters’s ability to digest a scouting report and adjust it as needed.

“That’s where he’s really good,” said Scherzer, who can be hard to please when it comes to pitch-calling matters. “He’s really good about understanding where we’re at within a report, where we’re at within a game, where we’re at here. When we need big pitches, he knows, this is the big pitch … He has a very good eye of understanding what they hitter’s trying to do as well.”

Wieters has an option worth $10.5 million for the 2018 season, one he seems likely to take given his down year and the state of the market. The Nationals can probably find more offensive production for less, though after a down year, perhaps Wieters will bounce back. The Nationals might have to consider back-up options in case he doesn’t. As they do, the Nationals will have to assess the value of some of his harder-to-quantify assets, like his pleasant presence in the clubhouse, and his rapport with an ultra-talented — and therefore, sometimes stubborn — pitching staff.

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