Capela, the NBA's leading rebounder and the team's third-leading scorer, has a back contusion. The Hawks announced Thursday that Young, their leading scorer and the league's No. 3 leader in assists, would miss the game with a left ankle sprain.
Interim coach Nate McMillan was counting on point guards Lou Williams and Brandon Goodwin and rookie centers Onyeka Okongwu and Nathan Knight to play significant minutes. Veteran big man Danilo Gallinari was a game-time decision with right foot soreness.
"It's a challenge because [Young and Capela] bring something you don't have," McMillan said. "What we've really had to do, as we've pretty much said, it's next man up. You don't have sitting on your bench what those two guys bring to the lineup as your starting point and your starting center and the way they've played this season. But when you have injuries such as we've had it's the next man up."
McMillan added that it's too early to tell how much time Young and Capela will miss. The Hawks hold the No. 5 spot in the Eastern Conference as they prepared to play the Heat, who are No. 7.
Atlanta has dealt with significant injuries this season. Starters De'Andre Hunter and Cam Reddish have missed multiple games as has reserve guard Tony Snell, at one point the NBA leader in 3-point percentage. Young, Capela, starting forward John Collins and reserve guard Bogdan Bogdanovic have been sidelined at several junctures, too.
Guard Kris Dunn has missed the entire season following right ankle surgery, but the Hawks are still hopeful he can return.