Broxton had an ultrasound exam that detected the swelling on Friday before the opener of the three-game series. He was listed as day to day.
Manager Dusty Baker wound up having to use Broxton to pitch the 14th inning of a 6-5 loss at Wrigley Field on Thursday. Broxton gave up a walk and a pair of hits, including Julio Borbon's RBI single. He fell to 2-2 with a 4.33 ERA in 29 appearances.
Baker said Friday that Broxton's elbow has been sore. He'd hoped not to use him against the Cubs.
"We were trying to wait to get back from Chicago to have him examined," Baker said.
The Reds signed Broxton to a three-year, $21 million contract in the offseason, planning to use him as either the setup man for Aroldis Chapman or the closer if Chapman should move into the starting rotation. Left-handed setup man Sean Marshall has been on the disabled list since April 10 with a sore shoulder, leaving the Reds scrambling for someone to pitch the eighth.
Baker has resisted bringing Chapman into games in the eighth inning. The left-hander went on the disabled list in 2011 with a sore shoulder and missed time near the end of last season because of a tired shoulder. Baker wants to limit him to one-inning appearances.
"He takes longer to get loose in the bullpen, and you don't have that much time," Baker said. "I don't know if you've noticed, but he's got a whole routine he goes through. If I want to use him in the eighth, he's got to start warming up in the seventh, and then what do you do if you don't use him?
"He's like a high priced sports car. It takes longer to get loose."
Outfielder Chris Heisey worked out Friday at Great American Ball Park, trying to get ready for a rehab assignment. Heisey has been on the disabled list since April 29 with a pulled hamstring. Twice he appeared to be close to returning, but aggravated the injury.
He'd hoped to start a rehab assignment a week ago, but the hamstring was sore after he practiced sliding, forcing the Reds to delay it.
"I'm really scared," Heisey said. "It's been seven weeks. There's definitely some apprehension. I'm feeling really good, as good as I've felt in a long time. I'm itching to get back. It's been long enough."
Cleanup hitter Ryan Ludwick, who tore cartilage in his right shoulder while sliding on opening day, played catch on Friday. He's hoping to return in the second half of the season.
"I think we're definitely on schedule," Ludwick said.