Ramirez, 24, is making his sixth tour with the big-league club and will be making his 14th start. He beat the Angels in the season's second game but is 0-5 with a 4.24 ERA in 12 starts since that victory.
“We've called him up a couple of times,” manager Lloyd McClendon said. “He's pitched well for us. He's given us opportunities to win ballgames. He's probably our most experienced guy down there.
“He's throwing the best down there. This is an opportunity that he's certainly earned.”
Ramirez is 5-5 with a 3.63 ERA in 14 starts at Tacoma, including 3-1 and 1.93 in his last five starts.
The Mariners required a starter for Wednesday after McClendon opted to take advantage of Thursday's open date to give every member of his rotation two extra days of rest prior to their next outing.
“This is how I explained it to them,” McClendon said. “‘Listen, I know it messes your routine up a little bit.' Not a lot, because they can double-sideline or whatever. A lot of times, Felix (Hernandez) doesn't even sideline.
“But I said, ‘Last month, I managed for September. This month, I'm managing for October.' I'm trying to keep the big picture in mind. This is absolutely the right thing to do.
“If we go to where we hope we can go, those innings will get way up there. When you have an opportunity to freshen them up like this, you have to take advantage of it.”
The Mariners must make a corresponding roster move to clear space for Ramirez. But any player optioned to the minors isn't likely to be affected by the 10-day stay normally required before being eligible for a recall.
Players can be recalled as soon as their minor-league club completes its season. Unless Tacoma can overcome a five-game deficit with seven games remaining, its season will end on Monday.
The Mariners' revised rotation positions Hernandez, Chris Young and Hisashi Iwakuma to pitch this weekend against Washington. Roenis Elias, James Paxton and Hernandez will pitch Monday to Wednesday at Oakland.
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