The Braves opted for Elder, a first-time All-Star, over rookie AJ Smith-Shawver or a bullpen opener. The Phillies are starting Aaron Nola (12-9, 4.46 ERA) in the teams' best-of-five NL Division Series, which is tied 1-1.
"I think we just kind of felt like after the way he threw at the intersquad and with the layoff, that he was the guy," Braves manager Brian Snitker said Wednesday. "He's been a starter for us all year. It was a big reason why we won the decision. We liked what we saw."
Elder won 12 games this season, but he appeared to tire down the stretch after throwing a career-high 174⅔ innings -- posting a 5.11 ERA over 68⅔ innings in 13 starts after the All-Star break. He especially struggled in his last three appearances, giving up 13 earned runs, 19 hits and 8 walks in 12 1/3 innings.
Overall, he was 12-4 with a 3.81 ERA and 128 strikeouts.
In his last appearance vs. the Phillies at Citizens Bank Park, on June 22, Elder allowed just three hits over seven scoreless innings, with six strikeouts. However, the Phillies roughed up Elder on Sept. 20 in Atlanta, chasing him as he allowed four runs in 3⅔ innings with five walks.
"When he's right, he's one of the best, most effective pitchers in baseball," Braves pitcher Spencer Strider said of his teammate. "He's confident. He's calm, and I think that this environment is going to bring the best out of him."
Elder was sent down to Triple-A Gwinnett during spring training, but he was a surprising success over the first half of the season after being recalled to Atlanta.
ESPN's Jesse Rogers and The Associated Press contributed to this report.