The Washington Nationals have placed Stephen Strasburg on the 15-day disabled list because of a strained muscle in his back.
The San Diego native skipped a scheduled bullpen session Wednesday.
The session was supposed to determine whether he could make a start on Saturday.
Strasburg was removed after two innings when he showed obvious discomfort during Friday's game against the Atlanta Braves.
The Nationals said the next day that Strasburg had a "slight strain" to the lat. The team was hoping he would not miss a start, although he was moved back two days in the rotation to give him time to heal.
Manager Davey Johnson said a pitcher will be promoted from Triple-A Syracuse to start on Saturday.
The San Diego State University alum is 3-5 with a 2.54 ERA.