Montero posted a .200/.231/.347 line with five doubles, two home runs and nine RBIs in 75 at-bats with Seattle. He had gone just 2-for-35 over his past 11 games after hitting .325 in his first 14 games following his promotion to the big leagues on July 10.
"It was an opportunity to get him some at-bats over the next six days and kind of sharpen him back up," said McClendon. "When he comes back, he'll have a better chance of helping us win ballgames. He hasn't played consistently. We had that stretch of lefties where he played, but now he's been sitting a little bit and he's gotten a little rusty."
Montero had seen his playing time diminish recently with Mark Trumbo heating up with eight home runs in his last 20 games entering play Wednesday. Trumbo and Montero have been splitting time at first base with left-handed-hitting Logan Morrison. Those three can also play designated hitter, but McClendon wants to use right fielder Nelson Cruz at DH at times as well to keep Cruz fresh, and that didn't leave many openings for Montero.
"You can only play so many guys," McClendon said. "Trumbo is really hard to sit right now."
Montero, 25, was named to the Pacific Coast League All-Star team at first base after batting .346 with 16 home runs and 75 RBIs in 93 games with Tacoma before being called up by the Mariners.
Montero is in his fourth season in the Mariners organization after being acquired in a trade for right-hander Michael Pineda in 2012. He was one of the top hitting prospects in baseball at the time as a young catcher for the Yankees, and he hit .260 with 15 homers and 62 RBIs during his rookie season in Seattle.
However, Montero ran into difficulties on and off the field over the next two years, drawing two suspensions and seeing his production dip in limited duty for the Mariners as they converted him to first base. Montero lost more than 30 pounds last offseason, and he impressed the Mariners with his renewed approach in Spring Training.
McClendon stressed that the latest decision wasn't about anything more than getting Montero back on track offensively.
"This is strictly a baseball decision to get him more at-bats," he said. "It was a smart decision. We can have him sit here for six days or go there and get possibly 30 at-bats. He'll be back on [Tuesday]."
Greg Johns is a reporter for MLB.com. Follow him on Twitter @GregJohnsMLB, read his Mariners Musings blog, and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.