Morrow, who had moved into the closer role with J.J. Putz on the 15-day disabled list (right elbow), is suffering from back spasms and didn't make the trip to Atlanta.
"We felt rather than make the long flight [on Thursday], he'd felt he might feel better in the next day or two and be able to get on a flight," interim manager Jim Riggleman said before his first game. "I'm not sure when that will be. I think it's literally day-to-day when he might be able to get on a plane."
Morrow hasn't allowed an earned run in his last 11 1/3 innings and hasn't been scored on in his past nine frames.He has struck out 15 and allowed just one walk and one hit during the scoreless streak.
The 23-year-old started the season in the Minors because of a tired right arm, but was recalled from Double-A Jackson on April 17.
Morrow, selected by the Mariners in the first round of the 2006 First-Year Player Draft, appeared in 60 games out of the bullpen last year as a rookie. He was 3-4 with a 4.12 ERA.
Guy Curtright is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.