The Mariners optioned the 25-year-old second baseman to Triple-A Tacoma before opening a series against the San Diego Padres and selected prospect Nick Franklin from Tacoma to take his place.
Ackley is the second young player the Mariners were counting on this season to get sent down, joining Jesus Montero. Seattle needed improvement from Ackley and Montero this season to get better offensively.
Instead, they're now both playing in the minors trying to solve problems at the plate.
"It was just time," Seattle manager Eric Wedge said. "Obviously, we fought for the guy for quite a period of time now and wanted to make it work up here and not have to do that. Ultimately, we couldn't take it any further."
Montero was sent down late last week after the club determined his career as a fulltime catcher was essentially over. The Mariners sent him to the minors to focus on hitting and learning the fundamentals of first base so he could be an option there.
The reasons for Ackley's demotion are similar - his struggles at the plate - but his play in the field remained strong despite his hitting problems.
Ackley was bothered by an ankle injury for much of last season, which led to fundamental breakdowns in his swing. Ackley kept tinkering with his swing early in this season, and he had appeared to overcome a slow first few weeks when he raised his average to .255 on May 4.
But his average plummeted from there, as Ackley collected just six hits in the next 16 games, hitting.111 during that stretch. He regularly fell behind in counts, and Wedge saw a progressive lack of aggressiveness in Ackley's swing.
Ackley was hitting .205 after Sunday, with just five extra-base hits in 45 games.
"Sometimes you get into a scenario as a big league hitter where you lose your feel, and I think that some of that was going on with him," Seattle general manager Jack Zduriencik said. "None of us have the exact answers. But we do know that if we remove him from the bright lights from the major leagues and let him go down to Triple-A and regain his stroke - again the history of what this guy has done in his career just stares you right in the face. That he's got to be a big league hitter. The issue is he hasn't been producing up here and he just needs to go back."
Franklin's promotion completes a rapid rise through the Mariners farm system. He was taken in the first round in 2009 out of high school and quickly made his way to Triple-A by the middle of last season. He was hitting .324 with four homers and 20 RBIs in 30 games with Tacoma this season.
He'll primarily play second base.
"I want to take a look at him. It's a great opportunity to take a look at him," Wedge said. "We'll let him get settled today and start him tomorrow."
Seattle also optioned reliever Lucas Luetge and recalled pitcher Hector Noesi from Tacoma. Noesi was brought in as protection in case Seattle needed a long reliever out of the bullpen, with struggling starters Aaron Harang and Brandon Maurer going the next two games and the Mariners playing 13 innings the day before.
The Mariners also designated for assignment minor league outfielder Francisco Martinez to clear a 40-man spot for Franklin, a move that makes Seattle's trade with Detroit in 2011 look even worse. The Mariners sent Doug Fister to the Tigers for three players, the only one of whom to make the big leagues is reliever Charlie Furbush. Meanwhile, Fister has become an anchor in the Tigers' rotation.