General Manager Mike Rizzo has some decisions to make before the July 31 trade deadline. (John McDonnell/The Washington Post)

When Max Scherzer went to the injured list Saturday with a mid-back strain that has lingered since early this month, the news set off a round of questions: How serious is the injury? How will the Washington Nationals line up their rotation until he comes back? Who are the team’s current fourth and fifth starters — after Stephen Strasburg, Patrick Corbin and Aníbal Sánchez — and who remains in the mix once Scherzer returns?

Those concerns are connected, centered on the Nationals’ lack of pitching depth, and they raise another possibility: Should they consider adding a back-of-the-rotation starter before the July 31 trade deadline?

The answer is yes, but with the caveat that Washington’s biggest issue remains its bullpen. Any mention of the Nationals and the deadline have focused on adding a reliever or two. And because their bullpen ERA is still the worst in the National League, that focus is justified. Yet they are still short a solid arm to round out one of baseball’s best rotations, and the trade market had its first domino fall Saturday.

The Boston Red Sox acquired 32-year-old right-hander Andrew Cashner from the Baltimore Orioles for two prospects. The Orioles, according to the Athletic, will pay half of Cashner’s remaining salary and pick up his $3 million signing bonus. It would have been, in immediate hindsight, a perfect deal for Washington to hunt: a reliable arm to fill out its staff, a trade partner willing to take on money, a way to add to a contending roster without blowing past the competitive balance tax. (We’ll get to that later.)

For now, with the Nationals cobbling together a short-term rotation, their depth options are Austin Voth, Erick Fedde, Kyle McGowin and Joe Ross, who has made 17 appearances out of Washington’s bullpen this year. Scherzer’s back strain aside, the Nationals need to pick a fifth starter out of this bunch. It was Jeremy Hellickson’s role going into the year, but he is on the 60-day injured list with a right shoulder strain. Voth had the most recent chance and followed a strong debut with two rough outings. Fedde was sharp in May, seemingly enough to earn his spot, until he faltered and was sent down. McGowin is currently in the bullpen as a long reliever. Ross is the only one of the four who is still across the country with Class AAA Fresno.

The Nationals started Saturday atop the NL wild-card standings and in second place in the division behind the Atlanta Braves. The 29-11 stretch to save their season was fueled by Scherzer, Strasburg, Corbin and Sánchez, to varying degrees. And it’s those pitchers who make a pennant chase seem possible. But the Nationals are 4-6 in games started by Fedde, Voth and McGowin, Those three have a combined 5.13 ERA in their starts. That, on its own, calls for the Nationals to take a critical look at the starting pitching market. Then factor in Scherzer’s back and consider that the last time Strasburg or Sánchez made 30 starts in a season was 2014, when Strasburg made 34 at age 25.

Washington, no matter what, will need some combination of Fedde, Voth, McGowin and Ross to shoulder a heavy load in the second half. But by strengthening its greatest strength, and investing even further in the rotation, that burden could be eased.

“As of right now, [the fifth-starter spot] is open,” Manager Dave Martinez said Friday. “We’ll see.”

With Cashner off the board, there are still many low-cost options available. Mike Leake (Seattle Mariners), Mike Minor (Texas Rangers) and Tanner Roark (Cincinnati Reds, and formerly with the Nationals) have been floated in rumors not related to Washington. That’s only the start of potential fits for a team looking to pad its roster for the stretch run. Other players will emerge depending on which teams decide to sell.

And if the Nationals want to get involved, that could require going over the competitive balance tax threshold. The organization has repeatedly stated that it wants to stay below that figure to avoid overage penalties. The Nationals are currently about $6.4 million under the threshold, according to Cot’s Baseball Contracts, and that doesn’t factor in end-of-year performance incentives that will get them much closer. It seems any deal worth doing, for a starter or reliever, or even both, would nudge the Nationals past it for a third straight season.

But the other option, as far as the rotation is concerned, is to keep switching between four unproven pitchers and hope someone sticks. That’s a risk for a team with revived World Series aspirations. The Nationals have 18 days to decide whether they want to take it.

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