Jayson Werth is scheduled to have an MRI on his balky right hamstring on Thursday, Nationals Manager Davey Johnson said following Wednesday’s 3-1 win over the Detroit Tigers. Werth, who hasn’t played in six days because of the hamstring tightness and a sore left ankle, will also be examined by an internist because the outfielder has been dealing a stomach virus that could be contributing to dehydration and muscle cramping. Werth has been battling a stomach bug for about a week, Johnson said, and fluid loss could be cause for the cramping. Although Werth had been cleared by his personal doctor, the Nationals’ team doctor, Wiemi Douoguih, examined Werth before Wednesday’s game. Werth was put through tests by trainer Lee Kuntz and “ran pretty good today,” Johnson said. But when Werth finished, “he started getting some muscle spasms,” Johnson added. “So he was obviously getting dehydrated.” Johnson didn’t consider the new development a setback. He and the Nationals believed that the hamstring was simply minor tightness, but the lingering caused Johnson to fear it had been pulled. Earlier in the day, Johnson reiterated that Werth wasn’t headed to the disabled list. “I have a good feeling if they can get rid of the bug he’ll be alright,” Johnson said.