But the game plan changed on Wednesday.
"It is out of necessity that this move was made," manager Don Wakamatsu said. "I know we had talked about [Luke] French starting on Friday in Texas, but now it will be Morrow. This gives us more bullpen help."
Even with an expanded roster, the Mariners are short in long relief. Right-hander Chris Jakubauskas had the back of his right shoulder examined on Tuesday and although there is nothing seriously wrong, he still has a knot that needs to be worked out and he probably will not be available for at least another couple of days.
It also is important for the team to see what kind of success Morrow has in a starting role.
The plan all along was to promote the former first-round Draft choice earlier this week, when the Rainiers' season ended. But Tacoma erased a 7-1/2 game deficit with 17 games remaining and won the PCL's North Division.
After much consultation, it was decided that Morrow and the organization would be better off to have him remain in the Minors for at least one more start.
Wakamatsu said the organization needs to begin making plans for next season. How Morrow fits in going forward ranks right up there among the most important decisions that must be made during the offseason.
French, acquired from the Tigers in the Jarrod Washburn trade, has had lukewarm results in his seven starts with Seattle, posting a 3-3 record and 6.38 ERA.
"We have seen what French can do," Wakamatsu said.
Now it's time to see what Morrow can do -- at least for one start.
Asked if French would return to the rotation, Wakamatsu said, "I think a lot depends on Morrow's outing."
Morrow was 5-3, with a 3.60 ERA in 10 starts with the Rainiers after being optioned on July 11. In his last seven starts, Morrow was 4-1, with a 1.96 ERA, suffering his only loss in his last start when he was limited to four innings because of a pitch-count limit.
The 25-year-old competed for a spot in the rotation during Spring Training, but began the season as the closer because that's where he was needed the most at the time.
Back-to-back blown saves against the Rangers in April led to him being sent to the Minor Leagues to rebuild his arm strength so he could reach a 100-plus pitch count regularly.
Starting has been his preference all along. He started five games for the Mariners last season, including a memorable debut -- taking a no-hitter into the seventh inning against the Yankees at Safeco Field.
The Mariners would welcome a performance just like that on Friday night. And so would Morrow.