UPDATE: Strasburg was scratched with forearm tightness, manager Davey Johnson confirmed. He first felt it while playing catch on Thursday. Johnson described it as a "soft tissue" injury and said the level of concern is low. He thinks it might be related to working on a new pitch. Strasburg will rest for a few days and the tentative plan is to start September 19.
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James Wagner of the Washington Post notes the 25-year-old Strasburg felt tightness in his hamstring 11 days ago, but it's unclear if that's why he has been scratched. Strasburg spent a little more than two weeks on the DL with a lat strain earlier this season. He missed time with shoulder inflammation in 2010 before missing most of 2011 following Tommy John surgery.
In a career-high 170 innings across 28 starts this year, Strasburg has gone 7-9 with a 2.96 ERA (127 ERA+) and 1.04 WHIP. He owns a career 2.95 ERA (132 ERA+) in 421 1/3 innings since breaking into the big leagues in 2010.
The Nationals infamously shut Strasburg down after 159 1/3 innings last season in an effort to control his workload and protect his arm. He was a deserving All-Star and the team was eliminated in heartbreaking fashion in Game 5 of the NLDS.