Max Scherzer threw from as far as 75 feet Friday and felt good, but the Nationals want to give him a break and then see how he feels. (Jonathan Newton/The Washington Post)

PHILADELPHIA — Washington Nationals ace Max Scherzer will miss his scheduled start Sunday against the Philadelphia Phillies as he deals with “mid-back” tightness, Manager Dave Martinez said Friday. Veteran Aníbal Sánchez will start in his place. It’s unclear when Scherzer will make his next start, though Scherzer said his target of returning Tuesday for the series opener in Baltimore against the Orioles is “realistic.”

“This is not something to be overly concerned about,” the Nationals’ ace added after his team’s 4-0 win on Friday night. “I know the feeling of what it’s going to take to get back on the mound and get completely through the ball. Like I said, we’re only a matter of days. This isn’t a long-term injury.”

The Nationals are off Monday before playing two in Baltimore, and Martinez said his probable pitchers for both games are “up in the air.” On Friday, Scherzer threw from as far as 75 feet, felt good and later spoke with Martinez. Still, the Nationals decided to push Scherzer’s start back “a couple days and see how he feels.”

“You know Max; he always feels like he’s ready to go,” Martinez said. “But we have to do the smart thing here."

Scherzer said he first felt sore after his second-to-last start of the first half against the Detroit Tigers. He received treatment and felt good enough to take the ball last Saturday against the Kansas City Royals, striking out 11 in the 6-0 win. Later, though, Scherzer felt his back tighten and spoke with Nationals General Manager Mike Rizzo and Martinez. The 34-year-old right-hander later withdrew from the All-Star Game.

Scherzer said the tightness comes from his back trying to stabilize his scapula (shoulder blade) as he pitches. After the Royals start, he tried to play catch Tuesday in Cleveland for the All-Star Game but couldn’t. On Wednesday, he got an MRI, and it came back clean, which told Scherzer it was a muscular issue and nothing more.

Quickly, Scherzer started doing math on the number of days until his next scheduled start and the days he’d need in between his next bullpen session and realized “it just wasn’t going to happen before Sunday.” He felt as though he’d made progress over the past two days with treatment and set his aim for Tuesday.

“We’ve been kind of lucky to have the all-star break to get the rest that I need,” Scherzer said. “We know exactly what this is. It’s just a matter of getting this right and making sure it doesn’t linger any further in the season.”

If Scherzer starts Tuesday, he could still start next Sunday against the Atlanta Braves on the regular four days’ rest.

On Friday morning, the Nationals called up right-hander Kyle McGowin from Class AA Harrisburg, but he’s expected to work out of the bullpen. Two potential spot starters, Austin Voth and Erick Fedde, are in Harrisburg, while another, Joe Ross, is at Class AAA Fresno.

In his past nine starts, Scherzer has a 0.84 ERA with 94 strikeouts and just nine walks in 64 innings. He is regarded as one of the more durable pitchers in baseball, even in the back half of his career, and has thrown more than 200 innings in each of his past six seasons. The last time he made fewer than 30 starts was in 2008, when he was a rookie with the Arizona Diamondbacks. He leads the National League with 129⅓ innings, well on pace to break 200 if he remains healthy, so the Nationals are making sure they’re careful coming out of the break.

Despite this workload, Scherzer maintained everything else in his body felt great.

“The fact that I was able to go out there [against the Royals] and make a start and have something tight and not compromise my shoulder or elbow, that's” — Scherzer knocked on the wood of his locker —"what we were really concerned about.”

He motioned at the “other little muscles” in his back and he waved off anymore concern.

“Going forward, I absolutely trust everybody in the medical staff, their opinions, their diagnosis what this and how soon I should be ready to be back on the mound.”

Scherzer was originally scheduled to have eight days of rest, four more than normal, between outings. Now he’s guaranteed even more with the rotation set for the weekend series in Philadelphia.

"He’s been on an unbelievable run, and we’re just trying to take care of him,” Martinez said.

Read more on the Nationals:

The Nationals saved their season with a late run. Now comes the real test.

Mailbag: Nats look to stay hot as second half opens in Philadelphia

Nats place Jonny Venters on IL, shift pitchers

The 'M' in MVP should stand for Max