Published 5:26 p.m. ET May 15, 2019 | Updated 8:55 p.m. ET May 15, 2019ATLANTA (AP) — The Braves gave another top prospect his shot.Austin Riley wasted no time taking advantage of the opportunity.Riley homered on his second at-bat in the big leagues, launching a high fastball from Michael Wacha deep into left-field seats to give Atlanta a 2-0 lead over St. Louis in the fourth inning Wednesday night.The 22-year-old struck out swinging in his first trip to the plate, but he sent his father and about 40 friends and family — many of whom hastily made the trip from his native Mississippi — into a raucous celebration with a 438-foot drive the next time up.Riley was promoted from Triple-A Gwinnett after a dominating start in the International League, where he was hitting .299 and leading the circuit with 15 homers and 39 RBIs. A roster spot opened when center fielder Ender Inciarte was placed on the 10-day injured list with a sore back.While normally a third baseman, Riley recently started three straight games in left field for the minor league team. That’s where he made his major league debut, batting sixth against the Cardinals.Riley wasn’t expecting the promotion, knowing there was a logjam on the infield behind third baseman Josh Donaldson and first baseman Freddie Freeman.”I had no clue,” Riley said before the game. “I was kind of waiting my turn. I hate Ender went down last night, but I’m happy to be here and hopefully I can help out.”Ronald Acuña Jr. moved over to play center field, giving Riley a chance to get extensive time in left. He also can play third base when Donaldson needs a day off.Riley was a first-round pick in 2015 and joined a Baby Braves lineup that already included the 21-year-old Acuña, the 2018 NL Rookie of the Year, 22-year-old second baseman Ozzie Albies and 25-year-old shortstop Dansby Swanson.Donaldson has a one-year contract, and the promotion of Riley gives the Braves a chance to see whether he is ready to take over at third base on a full-time basis next season.He certainly looked like he was prepared during his time at Gwinnett, hitting 13 homers in his last 18 games — including four multi-homer performances. With Inciarte hitting just .218, there’s a chance Riley could be in Atlanta for the long term even if third base is not yet open.”I felt good down in Triple-A,” Riley said. “I was swinging the bat well. I worked on a few things, cut down on my strikeouts, made more consistent contact, brought the walks up a little bit. That was the goals I was going for.”Riley said he’s felt comfortable in left field during his handful of appearances.”Going from third base to the outfield is little bit more relaxing,” he said. “Everything kind of slows down a little bit. It’s not bad.”The Braves also designated struggling reliever Jesse Biddle for assignment. The left-hander was let go after surrendering three runs in one-third of an inning Tuesday night in a 14-3 loss to the Cardinals.Biddle, a former top prospect with the Phillies who was out of options, pitched well out of the Braves’ bullpen in 2018, going 6-1 with a 3.11 ERA and one save in 60 appearances. But he struggled with his control this season, surrendering 10 walks in 11 2/3 innings, and had a 5.40 ERA in 15 appearances.”He’s going to have an opportunity somewhere else,” manager Brian Snitker said. “He had a good year for us last year. But that was last year. He’s had some struggles. It was hard to cover it up and get him going.”Right-hander Touki Toussaint was recalled from Gwinnett to work mainly in long relief. He’s 2-0 with a 6.35 ERA in three previous appearances with the Braves this season.___Follow Paul Newberry on Twitter at www.twitter.com/pnewberry1963 His work can be found at https://apnews.com/search/paul%20newberry___For more AP baseball coverage: https://apnews.com/apf-Baseball and https://twitter.com/AP_SportsCopyright 2019 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.