"This will give him a chance to go down and refine some things," manager Don Wakamatsu said. "I have seen a tremendous curveball, but there hasn't been any consistency with it. With that pitch in his repertoire, I think we will see a pitcher who will have a lot of success up here."
Morrow, who started the season as the Mariners closer and then was moved into the rotation following back-to-back blown saves against the Rangers in Arlington in May, made his sixth start on Friday night and went away from the game plan in a 6-4 loss to the Rangers.
The Rangers are a fastball-hitting team and the plan of attack was to change speeds. But just about everything Morrow threw was hard and the high-powered Texas offense was ready.
"You have to change speeds to be effective up here, especially against a team like that," Wakamatsu added.
Morrow has an 0-4 record and 5.28 ERA in 22 appearances this season. His 45 strikeouts in 46 innings pitched is indicative of his stuff, but he also has 31 walks, another reason his pitch count has limited the length of his starts.
Wakamatsu would not second-guess the original decision to re-convert Morrow into a starter at the big league level, rather than send him to the Minors to build arm strength and confidence.
"I told him today that the ability to stretch out at the Major League level was a positive," Wakamatsu said. "He got his pitch count up and now he can go down with a better idea of what he needs to do.
"I still stand behind the decision to stretch him out here."
Morrow had been penciled in to start Thursday's game against the Indians in Cleveland in the first game after the All-Star break. That assignment now goes to left-hander Garrett Olson, followed by Felix Hernandez, Jarrod Washburn and Erik Bedard.
That's the rotation unless Hernandez pitches more than one inning in the All-Star Game on Tuesday night. In that case, Washburn and Hernandez would flip-flop.
A fifth starter would not be needed until July 25, when the Mariners play the Indians at Safeco Field.
Morrow would be one of the candidates to get that start, along with left-handers Jason Vargas, who was sent to Tacoma last week to fine-tune his game, and Ryan Rowland-Smith.
"We have a lot of options," Wakamatsu said. "What we do depends on the schedule and where we are and what happens a month from now. It's a nice problem to have, a surplus down there to choose from."
The July 31 non-waiver Trade Deadline also could become a factor in the decision.