Strasburg was put on the 15-day disabled list Monday with what the team called a right strained flexor tendon in his forearm.
After the initial MRI on Sunday, the team wanted him to undergo a more extensive test including an injection of dye.
"Hopefully the results will be good," Washington manager Jim Riggleman said. "We're just going to have to hold our breath until that time."
Washington will call up right-hander Jordan Zimmermann to take Strasburg's spot in the rotation Thursday against the St. Louis Cardinals. Zimmermann hasn't pitched in the majors this year after having Tommy John surgery last August.
Strasburg is 5-3 with a 2.91 ERA and 92 strikeouts in 68 innings this season. This is Strasburg's second trip to the disabled list. He was placed on the DL last month with inflammation in his right shoulder.
Zimmermann was 3-5 with a 4.63 ERA and 92 strikeouts in 91 1/3 innings as a rookie last season. He has pitched for Triple-A Syracuse since Aug. 1 after a minor league rehabilitation assignment.
"Everything has been as good as it can be in terms of health and his performance," Riggleman said. "Every precaution has been taken."
The team also announced that outfielder Josh Willingham had surgery on his left knee.