Stephen Strasburg, shown exiting a game last month, was placed on the disabled list Monday with a sore right elbow. (John McDonnell/The Washington Post)

Perhaps only one cruel fact of baseball life could ruin a fan’s love of the game: the fragile human arm.

On Monday night at Camden Yards, Washington Nationals right-hander Stephen Strasburg, whose career was endangered by elbow ligament replacement surgery in 2010, was due to face Baltimore’s Dylan Bundy, the hottest Orioles pitching prospect in the past decade, who missed multiple seasons with both elbow and shoulder miseries. The night’s theme: the sport’s uplifting comebacks. Can’t keep a great phenom down.

Then, a few hours before the game, Strasburg was unexpectedly put on the disabled list with a sore elbow. Yes, that elbow. A “new elbow” that is reconstructed is supposed to last about how long on average in MLB? Six years? Or is the consensus this week that it’s eight years? Though some last forever.

This is, of course, the same Strasburg elbow in which the Nationals invested $175 million — all of it guaranteed — just three months ago, gambling Strasburg’s arm has seven more useful years in it. The pitcher can opt out of his deal after the 2019 or 2020 seasons, if he chooses to become a free agent. But the Nationals, like all teams that sign long-term pitching contracts, have no “opt-out” clause for injury.

All any team can do is hire good medical staff, handle star pitchers carefully, perhaps take out an insurance policy to recoup some lost contract money in case of disaster and then . . . what? Sacrifice sacred chickens on a pyre of Louisville Sluggers on the pitcher’s mound at midnight on a Friday the 13th?

The reality is that teams, and their fans, can’t do anything about these scary announcements, which are usually quite vague because nobody really knows.

Strasburg’s move to the DL is purely precautionary, according to the pitcher himself, as well as Manager Dusty Baker and General Manager Mike Rizzo. Nothing to worry too much about. Nothing to see here. After starting the season 15-1, Strasburg got shelled in his past three starts. Wednesday in Colorado, he had the worst game of his life, including those played in the back yards of neighbors; he got five Rockies out and allowed nine runs. Were they “earned?” Well, the Coors Field walls are still shaking from the line drives that dented them. If baseball were scored by Olympic judging standards, emphasizing aesthetics, Strasburg might have been charged with 90 runs.

But don’t fret. Strasburg’s elbow is merely more sore between starts than normal; and he has had more difficulty getting full range of moment recently, too. So why take chances? Why risk injury?

“We’ve been monitoring it for a while,” Rizzo claimed. “We felt like the prudent thing to do — like we always have with our pitchers — was to give him this reset. We’re going to put him on the DL rather than pitch through some routine inflammation and soreness. Hopefully, in 15 days, he comes back stronger than ever and he can recharge and be really strong at the end.”

That implies great optimism about the prospect of an overwhelming Strasburg all ready to roll in October in the playoffs — where the Nats are currently at 99.2 percent odds to be the National League East champions. “I think it’s just . . . being a Tommy John guy, getting through the grind of the season . . . there’s just a little more maintenance that has to go with it,” Strasburg said, “and I’m still trying to learn these things.”

That’s probably right. But don’t lap up the happy talk too fast. No team is happy about putting a pitcher on the DL this close to the end of the minor league season because, if he has the slightest setback, where can he go for rehab starts?

Nationals starter Joe Ross, who went on the DL seven weeks ago with a shoulder injury and was expected back in the rotation by now, is currently in that predicament. He won’t throw his first full bullpen session until Tuesday. He is running out of days to reach full strength and places where he can go to prove he’s back.

The most likely scenario is that this will be a good break for Strasburg, a refreshing though unwanted vacation, just as taking a week off to rest a pain in the neck seems to have helped Bryce Harper. Ryan Zimmerman also just got back from the DL and is hitting better; if he goes nuts in October, it might be because of his freshness. But that’s not the only Strasburg scenario. Cross your fingers: 175 million times.

For four days, the Nats and Orioles will meet in what should be as fun and frisky a regional-rivalry series as you could plan for a set of beautiful late August weeknights. Yet both pitching staffs, like those on every team, are punctuated by key hurlers who have had one or more major arm surgeries. The Nats’ key setup man Shawn Kelley is being used more heavily than at any time in his career, even though he’s had two Tommy John surgeries. He is one pitch from “career over.” Is a 70-appearance regular season pace smart before Kelley even reaches October, when he is on the first season of a three-year deal?

Even healthy hurlers live under an existential cloud. Thursday’s Orioles starter, ace Chris Tillman (15-5), was pushed back before his most recent start because of a tender shoulder. On Saturday, he wasn’t sharp. He’ll be under the microscope here this week. But then, who isn’t?

From the middle of February through spring and summer, fans watch the game but also discuss and fuss about it. We love to analyze, second-guess and predict the future. After all, baseball is the sport that’s most open to inspection. It rewards our attention. And that captures our affections.

But in one crucial respect, baseball is almost completely closed, a mystery to us and a fickle manipulator of our emotions and opinions. One minute we think we know how things stand. For instance, when the sun rose Monday, the Cubs were probably the best team in MLB, but the Nats looked like, perhaps, the second best .

One minute the Nationals’ sky is blue. Since June 26, they have the best record in baseball. In their first 122 games, they’ve outscored their foes by more runs (142) than they ever have in a full 162-game season.

Then, in the midst of the anticipation for this series, with Baltimore in a deep hitting slump and the Nats’ bullpen frazzled, with so much baseball to discuss in the stands at both Camden Yards and Nationals Park, baseball’s other reality intrudes.

We think we know the game. We almost start to trust it because it lets us learn so much about it and because we know the personalities of its players so intimately, like everyday acquaintances.

Then five words — “Strasburg goes on the DL” — remind us that nothing in baseball is more important than the sport’s single most unpredictable factor. When the games matter most, when the stakes are highest and the lifelong memories are made, which team’s star pitchers will even remain standing?