From June 15-Aug. 9, Broxton was on the DL with a flexor mass strain in his right elbow.
"We had just gotten big Broxton back," Reds manager Dusty Baker said. "He said he was feeling great. He was throwing the ball 97-98 [mph]. It's kind of a downer. You can get down, but you can't stay down. You've got to figure out a way to keep rolling. This is a pretty big blow. I was telling the other guys we have to pick it up as a team."
During the eighth inning of Wednesday's 10-7 win over the D-backs, Broxton allowed a Gerardo Parra homer, followed by a four-pitch walk to Wil Nieves. That's when Baker, pitching coach Bryan Price and trainer Paul Lessard visited the mound and took the reliever out.
Broxton, in the first season of a three-year, $21 million contract, will undergo an operation that would likely mean the end of his season. But Baker would not speculate on any timetables.
"They don't know until they get in there and see what's wrong," Baker said. "I'd just be guessing right now until the doctors get in there."
In 34 games this season, Broxton is 2-2 with a 4.11 ERA, 27 hits, 12 walks and 25 strikeouts over 30 2/3 innings.
Christiani was 6-5 with a 4.05 ERA, 15 walks and 48 strikeouts in 47 games this season for Louisville. But he allowed only three of his 23 inherited runners to score. Over his last 17 appearances, the 26-year-old had a 2.79 ERA. The Reds chose him over right-hander Curtis Partch, who had been up earlier this season.
"We've seen him in Spring Training," Baker said of Christiani. "I heard he's throwing the ball pretty good down there. Partch was the other candidate. He just got back himself from [the DL because of a biceps injury]."
Mark Sheldon is a reporter for MLB.com. Read his blog, Mark My Word, and follow him on Twitter @m_sheldon. Jeremy Warnemuende is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.