The Mariners have 10 days to trade, release, option or assign Baek outright to the Minors. He must clear waivers before being sent to the Minors, something the Mariners have been reluctant to do.
The Mariners didn't immediately announce who will replace Baek on the 25-man roster, but it probably will be right-handed knuckleball specialist R.A. Dickey, who has a 2-5 record and 3.44 ERA in seven starts with Triple-A Tacoma.
Whoever takes Baek's spot figures to also replace him as the long reliever, the most thankless job on any pitching staff -- something Baek learned the hard way this season.
A starting pitcher for most of his career, the 27-year-old South Korean was the 12th pitcher on the Mariners' 12-man pitching staff. He usually pitched when a starting pitcher was knocked out of the game early, or in blowouts when other relievers needed a day off.
"It's an unfair situation, but it is what it is," manager John McLaren said. "The long guy comes up short more than anybody else."
Baek entered Tuesday night's series opener against the Tigers in the fourth inning, replacing starter Carlos Silva, and was hit hard during a two-inning stint. He surrendered six hits, including three home runs and four runs in Seattle's eventual 12-8 loss.
He went into the game working on a streak of four consecutive scoreless innings over two appearances.
But whether he pitched well or poorly, the Mariners lost. They were 0-10 in games in which he appeared.
"The long man is there as an insurance policy," McLaren said. "He can sometimes go two weeks without seeing any action."
The less a long reliever is used, the better, because it means the starters are taking games into the late innings.
"You could say that," said McLaren when asked if some pitchers are sacrificed for the needs of the team. "Usually, a long reliever is a guy who can give you innings. That usually is a former starter or somebody who can't crack your rotation."
While pinch-hitting is regarded as the most difficult job for a position player, long relief is the toughest job for a pitcher.
"The long guy has to do a lot of work in the bullpen to stay sharp," McLaren said.