Following one collegiate season at Kentucky, Herro was selected with the 13th overall pick by the Heat in the 2019 NBA Draft. The rookie shooting guard made his NBA debut Oct. 23 in a win over the Grizzlies, recording 14 points, eight rebounds, two steals and one assist. Less than a week later, Oct. 29 against the Hawks, Herro posted his first 20-point effort: 29 points, seven rebounds and two assists. Against the Pacers on Dec. 27, he grabbed a season-high nine rebounds, adding 10 points and four assists in 22 minutes. Herro pocked up a season-high three steals against the Kings on Jan. 20, A week later in a win over the Magic, Herro blocked a season-high three shots in a start, adding 13 points, four rebounds and one assist. In the NBA bubble on Aug. 8 against the Suns, Herro racked up 25 points and handed out a season-high 10 assists for his first career double-double; he added eight rebounds as well. Four days later against the Thunder, Herro scored a season-high 30 points accompanied by six rebounds, two assists, a steal and a block. During the first three rounds of the playoffs, Herro averaged 16.5 points, 5.5 rebounds and 3.9 assists while making a name for himself with his clutch three-point shooting. That included an incredible performance in Game 4 of the Eastern Conference Finals against Boston in which he recorded 37 points, six rebounds and three assists with just one turnover. In the NBA Finals against the Lakers, the rookie averaged 14.7 points, 4.2 rebounds and 3.0 assists. His best Finals effort came in Game 4, when he managed 21 points with seven rebounds and three assists.