"I was out there running. I don't know why. It flared back up. I tried to get it adjusted. It was the first rib that caused the tightness," Strasburg said. "I had it adjusted today [by a chiropractor]. I have to keep my fingers crossed.
"I was pretty beat up this time. We went through all the different treatments and stuff, trying to get it working and get it normal. I wasn't really feeling progress. It was pretty sore from all the treatment."
With Strasburg out, right-hander Tanner Roark started the finale and pitched seven solid innings to help the Nats end a seven-game slide. On Friday, it looked as if Strasburg would make his scheduled start after he threw a light bullpen session that day.
"It was some kind of freak thing that must have gone on in the weight room," Strasburg said Friday. "I really don't know why. It kind of threw everything out of sync."
Strasburg also missed his previous start on Monday in Los Angeles because of the back issue.
Strasburg is off to the best start of his career, going 10-0 with a 2.90 ERA and 118 strikeouts in 93 innings.
"There is still a lot of baseball to play. I know the team needs me in big spots. I'm going to keep working at it and get back as soon as I can," Strasburg said.
With Strasburg on the DL, manager Dusty Baker said right-hander Joe Ross will pitch Monday against the Mets. Right-hander Lucas Giolito, the No. 1 prospect in baseball according to MLBPipeline.com, will be promoted to make his Major League debut on Tuesday.
Giolito was in his second season with Double-A Harrisburg. The team monitored his innings to start the season, but since May 25, Giolito has gone six or more innings in four out of his past six outings.
Bill Ladson has covered the Nationals/Expos for MLB.com since 2002 and writes an MLBlog, All Nats All the Time. He also can be found on Twitter @WashingNats. This story was not subject to the approval of Major League Baseball or its clubs.