Furbush, 26, had been on the disabled list since July 18 with a strained left triceps. He made five rehab appearances with Tacoma, posting a 6.00 ERA (four earned runs in six innings) with seven strikeouts, including two scoreless innings on Tuesday in Nashville.
"I'd never had an injury like this, so there wasn't an exact time table, it was just a day to day thing and finally I got to point where I just felt like myself again," Furbush said. "The one before last I felt good and I wanted to throw again, so that last game I threw two innings in Nashville and felt good."
Furbush, acquired as part of the Doug Fister trade with the Tigers a year ago, has been one of the Mariners' top relievers this season with a 4-2 record and 2.17 ERA. He's struck out 47 in 37 1/3 innings and has held left-handers to a .155 batting average.
Kelley, 28, was 2-3 with a 3.41 ERA, pitching 34 1/3 innings in 36 appearances.
The Mariners chose to keep veteran Josh Kinney and rookies Carter Capps and Stephen Pryor on the 25-man roster as their right-handed relievers, along with closer Tom Wilhelmsen.
Kelley started the season in Tacoma and still has options left, while Kinney is out of options and would have been exposed to the waiver process if he had been sent down.
"He had options and some of these other younger players that have options we wanted to continue to see, so he was the odd man out," said manager Eric Wedge.
Furbush's return gives Wedge three lefties in the pen, along with Oliver Perez and Lucas Luetge.
Furbush's only regret? He had to watch Felix Hernandez's perfect game Wednesday on a TV in the Nashville clubhouse.
'I just did what I needed to do to get back to 100 percent and unfortunately I missed it by two days," Furbush said with a grin. "It was unbelievable what Felix did. I'm real happy for him. We were watching it and a couple guys were talking about it and in my head I'm thinking, 'Can you jinx it if you're not there and you're talking about it?
"It was so cool. It's something so rare and so special, we were just happy for him. I just gave him a big hug today and said congrats. It's been a crazy year here at Safeco."