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The Nationals placed ace Stephen Strasburg on the 15-day disabled list with a Grade 1 right lat muscle strain in his lower back, the latest and potentially the most damaging blow to a struggling contender besieged by injuries.

Strasburg exited his start Friday in Atlanta after only two innings when he aggravated the lat muscle, which had been experiencing tightness for several starts. He would have pitched Saturday, but instead veteran right-hander Ross Ohlendorf will likely be called up from Class AAA Syracuse, according to multiple people with knowledge of the situation.

Strasburg will be eligible to come off the disabled list June 16. Lat strains often take between three and six weeks to fully heal, but Manager Davey Johnson said Strasburg could return from the DL on or shortly after the first day he is eligible.

“You don’t want anybody to pitch with an injury,” General Manager Mike Rizzo said. “Until he’s symptom-free, we’re not going to let him pitch.”

Strasburg had been scheduled to pitch a bullpen session this afternoon after playing catch from as far as 120 feet the previous two days. But because he still felt soreness in his lat, the Nationals canceled the bullpen session and decided to place him on the disabled list.

“He’s less and less sore each and every day,” Rizzo said. “But he still felt there was a little part of his lat that he was still sore.”

Strasburg still played catch today, warming up with the rest of the Nationals’ staff in right field before batting practice. As Nationals hitters took swings, Strasburg fired a ball off the side wall in right field, playing catch with himself.

Nationals officials had expressed confidence Strasburg could make the start Saturday for several days. Strasburg himself said he was moving “in the right direction.” An outside medical official called the timetable “overly optimistic” based on the usual timetable to return from a lat injury. Because Strasburg still felt stiffness in his lat today, the Nationals finally placed him on the disabled list.

“We were going to be cautious with him,” Rizzo said. “All along, we thought this injury was usually a DL-type of injury. Because he felt good, we wanted to see where he was at.”

The most pressing concern the Nationals face in regard to Strasburg’s injury is what could happen to his prized arm. Throwing with a lat injury can cause pitchers to overcompensate and lead to elbow or shoulder trouble. The lat muscle is crucial in the acceleration of the arm as a pitcher delivers a pitch.

“Any time a starting pitcher has something in the upper torso, it’s concerning,” Johnson said. “Not so much that injury, but what it might to do his arm.”

Strasburg, 24, is 3-5 with a 2.54 ERA in 12 starts. Last season, the Nationals shut him down in September in order to protect him from future arm injury in the wake of his 2010 Tommy John elbow ligament-replacement surgery.

In his place Saturday will likely be Ohlendorf, whom the Nationals signed this winter to a minor league contract. Ohlendorf last pitched May 30, and the Nationals skipped him when his next turn in the rotation came, a few days after Strasburg’s injury. Ohlendorf is 4-5 with a 4.32 ERA in Syracuse. In his past two starts, Ohlendorf has allowed one earned run over 12 1/3 innings with 22 strikeouts and seven walks.

Ohlendorf, 30, has made 73 starts in his six-year major league career. He made eight start last season for the San Diego Padres. In 22 appearances over the past two years, Ohlendofr has a 7.94 ERA over 87 1/3 innings.

The Nationals could “possibly” call up a reliever to take Strasburg’s roster Thursday and Friday,  Rizzo said. The two likeliest possibilities are lefty Xavier Cedeno, who spent one day with the Nationals last week, and right-hander Michael Crotta. Either pitcher would head back to Syracuse on Saturday to make room for Ohlendorf.