LOS ANGELES -- The Washington Nationals have scratched unbeaten right-hander Stephen Strasburg with an upper back strain, spoiling his scheduled showdown with Clayton Kershaw.

The NL East leaders announced the move about 80 minutes before the first pitch against the Los Angeles Dodgers on Monday night.

Yusmeiro Petit got the start in place of Strasburg (10-0), who was due to face Kershaw (10-1) for the first time in the aces' major league careers.

Strasburg was outstanding in his most recent start last week, pitching seven innings of one-run ball against the Chicago Cubs to win for the fifth time in seven starts. He has 118 strikeouts while allowing opponents to hit just .220.

Kershaw leads the majors in ERA (1.58) and strikeouts (133).