Johnson said Thursday he delivered the news to Strasburg on the plane ride home from Wednesday's win over the Miami Marlins.
Johnson says he has a "pretty firm plan" for Strasburg, but it's based on how many innings the right-hander pitches in his next starts.
Strasburg is 15-6 and has thrown 150 1/3 innings. The Nationals have not announced the innings limit they've imposed on him in his first full season since undergoing Tommy John surgery.
Barring a rainout or other change in the rotation, Strasburg's final start appears set for either Sept. 7 vs. Miami or Sept. 12 at the New York Mets.