Right-handed reliever Kameron Loe was designated for assignment by the Mariners to make room for Harang on the 40-man roster. Loe surrendered six home runs in 6 2/3 innings during four appearances for Seattle after making the club as a non-roster invite.
Harang is expected to join the Mariners on Friday. He hasn't pitched since Spring Training, when he was with the Dodgers, but Mariners general manager Jack Zduriencik said Harang threw a simulated game on Wednesday at San Diego State and will be given a close look in a bullpen session on Friday at Safeco Field.
"We have to get a feel for where he's at physically," Zduriencik said. "We have starters, so the fact he's going to bump somebody right now, I think is a little premature. But he is a starting pitcher and if he's in good shape and his pitch count is where we think it needs to be and his bullpen is successful, we'll address it after that."
The Mariners' rotation currently includes rookie Brandon Maurer and young right-hander Blake Beavan, who have both struggled in their first two starts. Zduriencik indicated one of the two could go to the bullpen in a long-relief role if Harang does take a starting spot.
The Mariners have some quality pitching prospects at Triple-A Tacoma, including 2011 first-round Draft pick Danny Hultzen and right-hander Erasmo Ramirez, as well as veteran Jeremy Bonderman. But Harang provides immediate depth to a rotation topped by Felix Hernandez, Hisashi Iwakuma and Joe Saunders.
Zduriencik doesn't want to rush Hultzen, while Ramirez is on the disabled list with a minor triceps strain and Bonderman is working to build himself up after being out of the game since 2010. Zduriencik had been talking to the Dodgers about Harang during the offseason, but the deal came together after he was dealt to the Rockies last weekend.
"Our biggest need came about when Erasmo wasn't able to step up," Zduriencik said. "He's going to miss a little bit of time here on the DL. And we didn't think Jeremy was ready to come here yet. He still needs time to pitch in Triple-A. So you have your ears open.
"Obviously with those two not available right now, we felt somebody like Aaron would fit well. If we'd have been healthy, maybe we wouldn't have done this. But more than anything, it's tough to find starting pitching and to have a guy like this available with his experience, we felt it was the right thing to do."
Harang, 34, is under contract for $7 million for 2013, plus either a $2 million buyout for 2014 or a mutual option for between $7-8 million depending on how many innings he pitches this season.
Harang was 10-10 with a 3.61 ERA in 179 2/3 innings with the Dodgers last season, then was traded to Colorado on Saturday for catcher Ramon Hernandez, with the Dodgers including $4.25 million to offset some of Harang's salary.
The Rockies immediately designated Harang for assignment and began shopping him, with several clubs expressing interest.
Harang has a career record of 105-104 with a 4.19 ERA in 299 appearances (293 starts). He pitched eight seasons for the Reds from 2003-10 and was their Opening Day starter his final five seasons, then was 14-7 with a 3.64 ERA for the Padres in '11 before signing as a free agent with the Dodgers last year.
Harang was the Reds' nominee for the Roberto Clemente Award in 2007 and '09, the Branch Rickey Award in '07 and the Marvin Miller Man of the Year from '07-09.
Hensley, 26, had appeared in two games in relief for Triple-A Tacoma this season, allowing one earned run on two hits and two walks in 2 2/3 innings. He'd been a starter most of his Minor League career, but was moved to the bullpen last year and posted a 4-6 record with a 4.84 ERA in 44 appearances for Tacoma and Double-A Jackson.