Strasburg, 28, was scratched from his start Monday night against the Baltimore Orioles.
Strasburg has struggled in his last three starts. He gave up seven runs in the first inning last Wednesday against the Colorado Rockies and was pulled in the second inning. In his last three starts, Strasburg posted a 14.66 ERA and allowed 24 hits in 19 2/3 innings.
Strasburg is 15-4 with a 3.59 ERA in 23 starts this season. His second stint on the disabled list this season is retroactive to Aug. 21.
Strasburg missed 17 games with an upper back strain, from June 16-July 3.
Right-hander A.J. Cole was promoted from Triple-A Syracuse to start Monday night against the Orioles.
Cole, 24, is 8-8 with a 4.26 ERA in 22 starts for Syracuse this season. He has a 5.79 ERA in three appearances (one start) for the Nationals, all recorded last season.