The move is retroactive to June 16 and comes after the right-handed Strasburg missed his second consecutive start.
Strasburg told reporters on Friday that he was injured during a weight room workout.
Strasburg said two ribs were out of alignment and later adjusted back into place.
He attempted to throw Saturday and felt discomfort and was scratched from Sunday's scheduled start.
Strasburg is 10-0 with a 2.90 ERA in 14 starts for the Nationals. He is the first National League starter to begin a season with 10 straight wins since San Diego's Andy Hawkins in 1985.
In May, Strasburg signed a seven-year, $175 million contract extension with the Nationals.
Tanner Roark started in place of Strasburg on Sunday against the Milwaukee Brewers.