Medlen had an MRI on Monday that revealed the injury, according to general manager Frank Wren.
Wren said Tuesday the 28-year-old right-hander and the team will seek a second medical opinion.
"I talked to him and he seems like he's in good spirits," Braves manager Fredi Gonzalez said.
Medlen felt the injury on Sunday while pitching against the New York Mets. He grabbed the elbow with two outs in the fourth inning and left with a trainer.
Medlen tore his right ulnar collateral ligament while pitching against the Mets on Aug. 4, 2010, and was operated on by Dr. James Andrews that Aug. 18. Medlen didn't pitch for the Braves again until Sept. 25 the following year.
He is expected to meet with Andrews this week.
Brandon Beachy, another right-hander in Atlanta's projected starting rotation, left a game Monday because of tightness around his pitching elbow. Beachy has been limited to a total of 18 starts over the past two seasons.
Left-hander Mike Minor, who underwent urinary tract surgery on Dec. 31, is struggling with shoulder soreness. He hopes to pitch within the next week but could open the season on the disabled list.
Atlanta could start the season without the trio, and Wren described the latest events as "worrisome.
Julio Tehran hasn't allowed a run all spring training after pitching four shutout innings against Philadelphia on Tuesday and is likely to be joined in the starting rotation by Alex Wood, who pitches Wednesday against Washington in a split-squad game. Rookie David Hale was impressive during a call-up to the Braves last September, but he has struggled a bit this spring training.