The 30-year-old right-hander allowed just one run in 15 outings, covering 23 2/3 innings of relief with the Bulls this season and compiling a 1-0 record with a 0.38 ERA. He had eight saves in eight opportunities for Durham.
"From his perspective, when we sent him out [at the end of Spring Training] we told him how much we liked him ... and that we would give him an opportunity if it presented itself, and it's presented itself," Rays manager Joe Maddon said.
Balfour said he built up his confidence while at Durham.
"Pitching is about confidence," Balfour said. "Get on a little bit of a roll and take it from there. Just go out there and drop a zero. Then go out there and drop another one. Like hitters getting hits. I just tried to roll with it as long as I could."
Balfour enjoyed closing for Durham.
"Everyone loves to pitch in close games," Balfour said. "I enjoy the pressure of key situations. I think you come in really zeroed in, really focused.
"You should be focused every time you go out there. But when it's 10-1, 11-1, it's different. Guys will tell you that. It's different when the game is on the line."
Balfour's role is yet to be defined.
"Just part of the [bullpen]," said Maddon when asked how he will use Balfour. "He can be used in multiple innings or he could be used to spot up later. But he's been closing down there and he's got closer kind of stuff. So we'll see."
The Rays acquired Balfour July 27 from the Brewers in exchange for right-hander Seth McClung. He appeared in 22 games with the Rays and struck out 27, walked 16 and posted an ERA of 6.14. Throwing strikes had been Balfour's biggest problem.
"That's the big thing about him," Maddon said. "It's been his lack of strike throwing, and now he is [throwing strikes] and we'll see if he can carry it over to the Major Leagues again. But physically we saw it in the past, excellent fastball, really good breaking stuff. And if he can locate with his pitches he can be very successful here."
Balfour is out of options, which means if the Rays want to send him out upon closer Troy Percival's return from the disabled list (left hamstring), he would have to clear waivers, an unlikely scenario given how well he has pitched at Durham. Nevertheless, the Rays did not take the safe route by recalling a pitcher with options, which sends the message that they are out to win by going with the best player for the job.
"He's been pitching at a very high level," Maddon said. "[Ten] walks [and] a bunch of strikeouts . He's pitched multiple innings also. He pitched up to three innings in one appearance this year, several times two innings, which is also very attractive. We were looking at the best possible fit for us at this time."
Bill Chastain is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.