(Gene Puskar / AP)

UPDATE, 11:40 p.m.: The Nationals are calling the Stephen Strasburg’s injury a right oblique strain. The right-hander is flying back to Washington. He said he wanted to stay in Friday night, but had felt the injury “nagging” his past few starts.

“Just a little tight back there,” Strasburg said after the Nationals’ 3-2 win. “It is what it is. I battled with it for a little bit the last few starts. Davey and Lee, they thought it was a good decision to get out of the game and not let it progress anymore.

“It’s something where, the last few starts I’d feel it warming up. I’d go out there and I wouldn’t feel anything. Tonight, it was more kind of like the reverse. I felt really good in the bullpen. I threw a couple pitches in there, and it started to tighten up. I felt it more and more. Nothing I could do about it. I’m kind of frustrated. You want to give at least seven, eight inning s a start. I’m just thankful Stammen was able to come in and pick me up and win the first one.”

UPDATE, 9:10 p.m.: Nationals ace Stephen Strasburg left the game with a muscular issue in his back, according to a person with familiar with the situation. It is not clear how long Strasburg dealt with the ailment, but Strasburg’s back had bothered him before tonight. It is not his arm or shoulder, which is relatively good news.

Strasburg exited the Nationals’ series opener tonight in Atlanta with an apparent injury after just two innings.

Strasburg appeared uncomfortable as he pitched through his second inning, which began with a deep home run by Freddie Freeman. Strasburg struck out the next hitter, Evan Gattis, twice hitting 98 miles per hour with his fastball.

In the next at-bat, against Brian McCann, Strasburg threw two consecutive changeups, the pitch that is hardest on his arm — it was the pitch he threw in 2010 when his ulnar collateral ligament snapped. On his next pitch to McCann, his fastball dipped to 94 miles per hour.

Strasburg threw five more fastballs, all of them hovering between 93 and 95  miles per hour. After several pitches, Strasburg had appeared to roll his right shoulder and grimace. McCann would ground out, and then Strasburg struck out Dan Uggla with a curveball to end the inning.

In the dugout, pitching coach Steve McCatty and trainer Lee Kuntz huddled with Strasburg. He stared glumly ahead, a pinch of chewing tobacco tucked in his lower lip, as McCatty talked to him. In the bullpen, Craig Stammen had already started to warm up. After the Nationals finished hitting in the top of the inning, Stammen jogged from the bullpen in right field.

Strasburg’s apparent injury adds to a calamitous first third of the season for the Nationals. They sat at 27-27 entering the weekend and have played chunks of the season without Bryce Harper, Jayson Werth, Ryan Zimmerman, Wilson Ramos and Ross Detwiler. Their pitching depth, a weakness to start the year, was depleted this week when veteran right-hander Chris Young went on the disabled list at Class AAA Syracuse.

Strasburg underwent Tommy John elbow ligament replacement surgery during his rookie year in 2010. Last season, General Manager Mike Rizzo controversially shut down Strasburg in September as the Nationals headed into the playoffs.

Earlier this season, Strasburg raised fleeting concerns about the health of his arm during a start in Atlanta. After he frequently shook his right arm on April 29, Manager Davey Johnson said Strasburg was feeling tightness in his shoulder. Strasburg and Rizzo insisted he was fine, and he has not missed any action since.

Entering tonight, Strasburg had reeled off one of the better stretches of his career. He twice pitched into the eighth inning – which he had never previously done – and allowed three earned runs in 23 innings across three starts. In those three outings, he threw 117, 108 and 112 pitches.

More when available.