Team medical director Dr. Tim Kremchek delivered the bad news to Guardado at his office on Wednesday morning.
"Just when he came in, his body language wasn't looking too good," Guardado said. "He told me and showed me the MRI. You could see the tear. He was saying some other stuff to me, but I was lost after that."
The Reds moved Guardado from the 15-day disabled list to the 60-day DL and called up veteran pitcher Jason Johnson to take his roster spot.
Guardado was acquired by Cincinnati in a July 6 trade with the Mariners for pitching prospect Travis Chick. The lefty, who had lost his closer's job in Seattle, immediately solidified the role with the Reds and helped stabilize a sagging bullpen.
In 15 appearances for the Reds, Guardado was eight of 10 in save chances and posted a 1.29 ERA. He was 1-3 with a 5.48 ERA and five saves in 28 games with Seattle.
"I'm at a loss for words," Guardado said. "No pitcher wants to go through something like this, especially coming to a new team. I'm trying to help this team out, and I can't. That's the frustrating part."
Nagged by muscle soreness in his left forearm since late July, there were occasions when Guardado was unavailable and had to be rested. Although an earlier MRI test revealed no tears, he finally went on the DL on Aug. 20, when he couldn't get through an inning the previous night.
Guardado had tried to rest his arm and then rehab it by playing catch in recent days, but he still felt soreness before agreeing to undergo more complete testing.
"As a pitcher and the person I am, you're hoping maybe just a couple of more days of doing what I'm doing [is] going to get [it] better," Guardado said. "But yeah, something was still there. It was driving me crazy. I didn't know where I was, and now I know."
It was another blow to the playoff hopes for a struggling Reds team, which has employed a closer-by-committee system in Guardado's absence. David Weathers, Rheal Cormier, Ryan Franklin and Scott Schoeneweis are all considered available for the ninth inning.
Guardado is expected to have Tommy John surgery to repair the ligament, but a date was not set. Rehabilitation could take up to one year. The 35-year-old planned to keep pitching after he recovers, but since he can become a free agent after the season, he may have thrown his last pitch for Cincinnati already.
"I'll be back. You're going to see me back," Guardado said. "Who knows? I told [the] doc, 'If you're going to do this, put a couple of more knots in there. Maybe, I'll be back to 95 [mph] or something.'"
Signed Aug. 30 to a contract with Triple-A Louisville, Johnson became the Reds' league-leading 35th player on the active roster.
The 32-year-old Johnson has strong local ties. He is a native of Hebron, Ky., and attended Conner High School.
"It's kind of good to be back in town," Johnson said. "It's familiar to me and comfortable."
Johnson will be in an unfamiliar situation, however. The long-time starting pitcher will be in the National League and be used in a long-relief role for the first time since 1997, when he was a rookie with the Pirates.
"It's something you have to adjust to," said Johnson, who came in with a 55-98 record and a 5.00 lifetime ERA. "[You have to] get loose and get ready quick. I'll have to get used to it and go out there and pitch."
Johnson was 3-12 with a 6.35 ERA in 20 starts for the Red Sox and Indians this season, including 0-4 with a 7.36 ERA in six starts for Boston before his release last month.
If the Reds have a lead to protect in the late innings, look for manager Jerry Narron to lift left fielder Adam Dunn from those games for better defense. In Tuesday's 3-0 win over the Giants, Dunn was replaced by Chris Denorfia in the eighth inning.
Since Dunn can change games with his bat, Narron was reluctant to remove him at any point of a game earlier this season.
"If we think for sure his bat isn't going to come up again, or it shouldn't come up, or if it does come up that it won't make a difference, that's a big part of it," Narron said.
Denorfia, Dewayne Wise, Norris Hopper and right fielder Ryan Freel are among the speedier options Narron has late in games.
"Those guys can flat-out run it down," Narron said.
The Reds are off on Thursday. Kyle Lohse (3-7, 5.63 ERA) will pitch on Friday at 7:10 p.m. ET in the opening game of a three-game series against the Pirates and right-hander Ian Snell (12-9, 4.83).