Morton, one of baseball's best clutch pitchers, will be looking to give the Braves a clinching victory in the best-of-five series.
He'll be opposed by Milwaukee's Eric Lauer, who is set to make his first appearance of this postseason. The left-hander went 7-5 with a 3.19 ERA in 24 appearances during the regular season, including 20 starts.
Both teams waited until just a few hours before the game to announce their starting pitchers on Twitter.
Morton worked into the seventh inning on Friday, throwing 85 pitches in a 2-1 loss to the Brewers in Game 1. He is the only Atlanta pitcher to give up any runs in the series, surrendering a two-run homer to Rowdy Tellez.
Atlanta bounced back with a pair of shutout victories to grab a 2-1 lead in the series. If the Brewers win Game 4, the deciding game would be held Thursday in Milwaukee.
Atlanta's Max Fried would start Game 5 on full rest if necessary. Otherwise, he would start Game 1 of the National League Championship Series against either the San Francisco Giants or the Los Angeles Dodgers.
The Brewers decided not to pitch ace Corbin Burnes on three days' rest in Game 4, which would make him available to start Thursday if the series goes back to Milwaukee.
Morton has pitched on short rest one other time in the postseason -- with Houston in 2017. He started Game 4 of the World Series that year, pitching 6 1/3 innings, then came back in relief in Game 7 to work the final four innings. He allowed just one run and earned the win in a Series-clinching 5-1 victory over the Dodgers.
Morton, who is 37, is the only pitcher in baseball history to win four winner-take-all games in the playoffs. He was 7-4 with a 3.34 ERA in 14 postseason appearances before Tuesday.
Morton went 14-6 with a 3.34 ERA this season. He recently agreed to a $20 million contract extension to return to the Braves in 2022.