The Braves bought out Heyward's last arbitration-eligible years on February 4, 2014, agreeing on a two-year, $13.3 million contract. Already rated one of the top defensive outfielders in the league, his coverage in right field significantly improved, demonstrated through an increased defensive runs saved (DRS) total. Through May 19, he registered 16 DRS, matching three of his previous four entire season totals. Inside Edge (IE) charted that of all batted balls hit to him, Heyward had missed a total of nine in 358 innings; all nine were rated as having a 10% or lower chance of being caught. Two catches in motion off the bat of Mike Trout – one tumbling on a sinking line drive and one sprinting and leaping at the warning track – helped ensure a 7–3 victory over the Los Angeles Angels of Anaheim on June 15. Heyward also added a home run.
On November 17, 2014, the Braves traded Heyward to the St. Louis Cardinals along with pitcher Jordan Walden for pitchers Shelby Miller and Tyrell Jenkins to replace their former right fielder and top prospect Oscar Taveras, who died in a car accident a month earlier. Cardinals manager Mike Matheny, who wore uniform #22 and had also done so for most of his playing career, gave his number to Heyward. Heyward wears this number to honor his friend Andrew Wilmot. After the trade was announced, Heyward published a Twitter message thanking the Atlanta Braves organization and fans for their support.
"The ball he hits well is the ball out away from him," according to the same scout. "If pitchers pitch around him, on the outer half, he'll crush it. ... That's why Niese hit him: He went up and in, and Heyward's a diver." Indeed, charts from STATS LLC depicting Heyward's "Hot Zones," or a hitter's zones of maximum effectiveness, corroborate the scout's findings. Through April 25, 2014, Heyward had swung at 22 of 55 pitches in 2014 that qualified as "up and in," crossing the plate either to the upper left ninth of the strike zone or slightly higher or inside of it. None of those 22 swings had produced a hit.
Despite not reaching his offensive projections, his defense has delivered as promised. Heyward has been rated as one of the top right fielders, if not the top right fielder, in MLB. The Fielding Bible staff has said that he "is the best defensive right fielder in baseball, bar none." He is an expert at instantly picking up and reacting to the path of the batted ball and following it with efficient routes. Such defensive ability was demonstrated on a fly ball off Justin Turner's bat during the 2013 season. The same play was part of a demonstration in 2014 of Major League Baseball Advanced Media's Statcast system, a hybrid of PITCHf/x, FIELDf/x, and a radar-based play tracking. Statcast analyzed that Heyward got a jump off the fly ball the bat in ⁄100 second, ran at 18.5 miles per hour (29.8 km/h) and took a route with a 97% efficiency. The ball had 4.0 seconds of hang time and he ran 80.9 feet (24.7 m) for the catch.
In the rating of three zones to where the ball is hit in the right field in his first five seasons, he was above average at saving bases on shallow-hit (+31) and medium-hit (+40) balls. His greatest strength came with deep-hit balls (+140). In 2014 alone, he saved +40 bases on balls hit to the deepest part of right field, which was the equivalent of 20 doubles. The enormous ground coverage makes up for what would be considered a slightly below-average arm among right fielders. Commented Fielding Bible founder John Dewan, "Heyward has been able to accomplish this (winning the Fielding Bible Award) by starting and finishing every play extremely well. He excels at picking up the ball off the bat and rarely takes the wrong angle. He is not afraid to dive, demonstrating tremendous body control when doing so."