Guerra, who was the Dodgers' Opening Day closer, was optioned to Triple-A on Aug. 21 after command problems earlier in the month. He made three appearances for the Isotopes and gave up four earned runs in 4 1/3 innings.
As closer Kenley Jansen waits to see how long he will be sidelined with the recurrence of an irregular heartbeat, the addition of Guerra will be a welcome one. He is 2-3 with a 2.66 ERA this season in 44 innings pitched.
"We wanted him to work on some things. In a perfect world he would have more time to do it," manager Don Mattingly said. "But we are at where we are at right now. Right now, we are OK in the back end, but you never know. It could be in the eighth or it can be early."
Federowicz will serve as the team's third catcher behind starter A.J. Ellis and Matt Treanor. The 25-year-old batted .294 with 34 doubles, 11 homers and 76 RBIs in 115 games this season with the Isotopes.
He went 2-for-13 with an RBI in seven games last season with the Dodgers.
"I think I'm a lot better," he said. "I had a good season offensively and defensively. I think I had one of my best defensive years. I've definitely improved a lot. Just getting reps, playing in games, that's the biggest thing. I'm very excited."
Ely comes to the Dodgers after a stellar season in the Pacific Coast League that earned him Pitcher of the Year. The 26-year-old had 14 wins, a 3.20 ERA and 165 strikeouts with the Isotopes this season. He led the PCL in strikeouts and ERA and he was tied for first in wins entering Saturday.
The righty has gone 4-11 with a 5.35 ERA in 23 MLB games (19 starts) since debuting in 2010.
"I'm driving my fastball better, not nitpicking at the edges. Hit it, this is what I got," Ely said. "It's made me a better pitcher."
After a promising 2010, Ely had a disappointing year in 2011 and was taken off the 40-man roster. Now that he's back with the Dodgers, Mattingly said Ely could be used as a spot starter if someone were to get hurt or as a long reliever.
"I worked hard in the offseason so I would hit the ground running in camp with a big head of steam. A chip on the shoulder is how I took it," he said. "I wanted to go into camp with a better mindset, be consistent, attack the hitters and it worked out all year. I'm glad I could reopen some eyes and get back to the Major League level."
Mattingly said he expects more players to be called up in the coming week and Bobby Abreu's name came up as a possibility.