Stephen Strasburg imploded after Ryan Zimmerman’s throwing error in the fourth inning of the Nationals’ 6-2 loss to the Atlanta Braves. (Jonathan Newton/The Washington Post)

Stephen Strasburg hardly encountered trouble Saturday night, until he found himself in the center of a crisis. He cruised through the fourth inning against the Washington Nationals’ primary antagonist, one out from another zero on the scoreboard. And then Andrelton Simmons smashed a groundball to third base, Ryan Zimmerman backpedaled and Nationals Park held its collective breath: sigh or groan?

Air gushed out of every corner of the stadium. Zimmerman’s parabolic throw over Adam LaRoche’s head derailed Strasburg, reignited concerns over the condition of the franchise third baseman’s right shoulder and provided the pivot for the Atlanta Braves’ 6-2 victory. The Nationals will wake up Sunday trying to salvage both their series against the defending NL East champions and the state, both physical and mental, of Zimmerman.

Zimmerman exited the game with soreness in his right shoulder, which spiked as he made the crucial throw, Manager Matt Williams said. Zimmerman will undergo tests, including an MRI, on a shoulder repaired by major surgery in November 2012. The Nationals hope to have results Sunday.

Until they know, Zimmerman’s health will remain their biggest concern — over the 3-for-21 slump that will likely keep Bryce Harper out of Sunday’s lineup and their continued struggle against the Braves.

“We need his glove, we need his bat in that lineup,” LaRoche said. “If it happens, it happens. Hopefully, it’s not too long. Early in the season, the most important thing is getting him healthy and getting him back out there. If he needs a few days or whatever it is, it’s not the end of the world. We’ll miss him, if in fact he’s out for a little while.”

A baseball veteran who knows no other life than the grind of the sport, third baseman Ryan Zimmerman has been with the Nationals since they came to Washington. He takes a pause during spring training to talk about the mental and physical challenges of the game. (Whitney Leaming/The Washington Post)

Five games into the six-year, $100 million contract extension he signed two years ago, Zimmerman’s arm has come into question. In the spring, Zimmerman sat out for almost a week with shoulder soreness. He wore a large ice pack daily. But he also believed it was a typical symptom, something that would dissipate. Now, the Nationals are alarmed. “Of course,” Williams said.

Zimmerman finished last season with his throwing woes seemingly conquered. But, for reasons he explained before Saturday night’s game, he could not simply pick up where he left off. Some days, his surgically repaired right shoulder cooperates. Other days, especially in the cold, and in spite of constant work, it will not.

“It’s definitely re-learning,” Zimmerman said Saturday afternoon. “It’s complicated. It’s hard to explain to people that have never played baseball. The best way to explain it is, some days are really good, and some days are really bad.”

Saturday was one of the bad days. Simmons’s groundball, which came with two outs in the fourth, the Nationals up, 2-0, and a runner on second, forced Zimmerman to take two steps back and field it with his backhand. Zimmerman had no momentum. All the power to get the ball across the diamond would have to come from his right arm.

Zimmerman launched the ball as much as he threw it. As it arced over the mound, LaRoche shuffled backward. He reached his right arm into the air, but the ball deflected off the tip of his glove. The ball hopped into the camera well. Justin Upton scampered home. Zimmerman kicked dirt and blew air into his bare hand.

“It’s just as hard for us as it is for him,” LaRoche said. “He’s our captain on this team. He’s a big piece of it, to say the least. To watch him go through that, it’s frustrating. We’re all behind him.”

Rather than settling into the dugout, Strasburg climbed the mound, needing to stem damage. The batter walking to the plate — Julio Teheran, the Braves’ starting pitcher — should have helped. Teheran, a .203 career hitter, lashed a 1-1, 94-mph fastball to right field. Simmons scored from second, tying the game at 2.

“That’s just kind of a stupid decision on my part,” Strasburg said. “I knew him well enough. He can hit a fastball.”

Once Zimmerman heaved the ball over LaRoche, Strasburg’s outing imploded. In the fifth, Freddie Freeman drew a one-out walk and the next four batters ripped singles. Williams trudged to the mound and took the ball with the Nationals trailing, 5-2, and two runners on base. The deficit would grow to 6-2 after a sacrifice fly.

Before Zimmerman’s error, Strasburg faced 16 batters and five reached base. After Zimmerman’s error, Strasburg faced eight batters. Six reached base and four scored.

“Just from last year, I want to make a conscious effort of not letting those things bother me,” Strasburg said.

Zimmerman has had two chances this season to make throws to first base when not charging a ball or on the run. Both throws have resulted in unsightly errors. Wednesday night in New York, a throw sailed so far to LaRoche’s right he could not reach the ball even after he shuffled off the base and dove. Even during warmups between innings, Zimmerman’s throws bounce or pull LaRoche off the base.

“It’s getting worse,” said one National League scout who watched the Nationals frequently during spring. Speaking candidly, Nationals officials acknowledged Zimmerman’s throwing issues. But they also saw progress, and believed his throwing, like last year, would come around. Then he suffered another setback.