Riley homered in his second at-bat in the big leagues, launching a high fastball from Michael Wacha deep into the left-field seats, to give Atlanta a two-run lead over St. Louis in the fourth inning of what turned out to be a 4-0 victory Wednesday.
Riley, 22, 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, recently rated baseball's No. 87 overall prospect by ESPN's Keith Law, was promoted from Triple-A Gwinnett after a dominant start in the International League, in which 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.
He hit another drive to the edge of the warning track his next time up, and handled a couple of chances without any problem in left. Normally a third baseman, he figures to mostly play left field at least during his initial stint in the big leagues.
"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 Acuna 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.
Donaldson has a one-year contract, and Riley's promotion 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. He hit 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 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 has felt comfortable in left field in 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 after the left-hander surrendered three runs in one-third of an inning in a 14-3 loss to the Cardinals on Tuesday.
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.
Information from The Associated Press was included in this report.