LOS ANGELES -- Los Angeles Dodgers reliever Hong-Chih Kuo was placed on the 15-day disabled list before Saturday night's game against the St. Louis Cardinals with a lower-back strain. Kuo had been bothered by back discomfort all season, a fact that might have affected his mechanics and contributed to his uncharacteristic four walks in 2 2/3 innings.
The move was made retroactive to Thursday, meaning Kuo will be eligible for activation as early as the team's April 29 series opener against the San Diego Padres.
Kuo underwent an MRI exam last weekend in San Diego that showed nothing serious.
"But we were wondering why he was up, up, up all the time," Dodgers manager Don Mattingly said. "He had trouble getting the ball down in the games and even in the bullpen. ... And then [Friday] night, he was trying to warm up and just wasn't able to get it going. So we decided we had to do something."
To replace Kuo on the roster, the Dodgers recalled reliever Ramon Troncoso from Triple-A Albuquerque, where he had allowed two runs on four hits over seven innings in three appearances this season, striking out six with only one walk.
Mattingly said Kuo is scheduled for a series of tests over the next couple of days, but that club officials are holding out hope that he will be able to return quickly. Kuo has a well-chronicled history of arm problems, including two Tommy John surgeries while he was still in the minors, but this appears to be unrelated.
"The good part is that it isn't the elbow or the shoulder," Mattingly said. "We feel like it's muscular. Hopefully, he can get it worked out and go from there."
By replacing the left-handed Kuo with the right-handed Troncoso, the Dodgers have left themselves without a single lefty in their bullpen.
"That is always a concern," Mattingly said. "You would like to have a couple of lefties so late in the game you can move guys around and go back and forth a little bit. But we wanted to go with the guy who was throwing the ball the best right now, and Tronco has been throwing it good, so he is the guy."
Mattingly said veteran reliever Matt Guerrier, who hasn't given up a run in 7 2/3 innings this season, will take over Kuo's eighth-inning setup role while he is out.
Tony Jackson covers the Dodgers for ESPNLosAngeles.com. Follow him on Twitter.