The left side of Strasburg's neck tightened after the first inning of the Nationals' 5-2 loss to the Cincinnati Reds on Friday night, and he left after facing one batter in the second inning.
Manager Matt Williams said he hasn't decided who will take Strasburg's spot in the rotation.
Strasburg is 3-5 with a 6.55 earned-run average in 10 starts. He injured his left ankle during a conditioning drill in spring training and the Nationals want to figure out whether his back and neck problems are caused by a change in his delivery to compensate for the ankle problem.
"Something's causing him discomfort in his upper back," Williams said. "The fact that it switched sides wouldn't point to anything structural. We've got to get it figured out."
Nationals' Bryce Harper sits out game
Outfielder Bryce Harper was scratched from Washington's lineup because of a sore back.
Harper was hit in the middle of the back by a pitch from Cincinnati's Tony Cingrani on Friday night.
Harper is batting .369 with 13 home runs and 28 runs batted in this month.
Marlins' Adeiny Hechavarria doesn't play
Miami Marlins shortstop Adeiny Hechavarria didn't play against the New York Mets because of a bruised left shoulder.
Hechavarria banged into left fielder Christian Yelich while they were pursuing Daniel Murphy's bloop double in the ninth inning Friday night. Hechavarria was shaken up but finished the game.
Hechavarria is hitting .300 with two home runs and 20 RBIs.
Rockies' Jorge De La Rosa is scratched from start
Colorado Rockies left-hander Jorge De La Rosa was scratched from his scheduled start Saturday against the Philadelphia Phillies because of a cut on the middle finger of his pitching hand.
De La Rosa (1-2, 6.51 ERA) is scheduled to start Tuesday's home game against the Dodgers.
Milwaukee Brewers reliever Will Smith will have a hearing Monday for his appeal of an eight-game suspension after being found with a foreign substance on his arm while pitching.
Manager Craig Counsell said that he wasn't sure how soon the team would know the outcome of the appeal.
Smith was ejected from Milwaukee's 10-1 loss in Atlanta on May 21 after Manager Fredi Gonzalez asked umpires to check the left-handed reliever.
Smith has said he put the substance on his forearm before warming up in the bullpen, and that he forgot to wipe it off after being rushed into the game.