DALLAS -- Mavericks superstar Luka Doncic sustained a moderate right ankle sprain Saturday in the opening minutes of Dallas' 122-118 overtime loss to the Miami Heat, league sources told ESPN's Adrian Wojnarowski.
X-rays on the ankle were negative, the team said.
"We hope it's not serious," said Mavs coach Rick Carlisle, who added that he does not expect Doncic to be available Monday night for a road game against the Milwaukee Bucks. "That's about all that we have for you now."
Doncic injured the ankle early in the game when he stepped on Heat guard Kendrick Nunn's foot while driving to the basket.
After not being able to run back on defense, Doncic left the floor during a timeout with 10:20 remaining in the first quarter. He hobbled to the locker room for further evaluation. He was ruled out by the end of the first quarter.
"Next man up," said Mavs shooting guard Tim Hardaway Jr., who had 28 points in the loss. "Luka is obviously a huge, huge part of this organization, this team. To see him go down is not easy to see. Just got to figure things out. ... Hopefully, he's back soon."
Doncic, the Rookie of the Year last season, has established himself as an early MVP candidate. The 20-year-old was averaging 30.4 points, 9.9 rebounds and 9.3 assists per game entering Saturday night and leads the league with eight triple-doubles, including a 41-point performance in Thursday's win over the Detroit Pistons.
Doncic's streak of 20 consecutive games with at least 20 points, five rebounds and five assists ended due to the injury. Only Oscar Robertson had a longer such streak in NBA history.
"If I had to watch somebody play and me pay to watch, he goes in that category," said Heat star Jimmy Butler, who had 27 points and seven assists in Miami's win.
Carlisle said Doncic, who left the arena without speaking to the media, was able to put weight on his right leg. Teammates told ESPN that Doncic was working out on the team's underwater treadmill under the supervision of the Mavs' athletic training staff at halftime.
The timing of Doncic's injury is especially tough for the Mavs, who had won 11 of their previous 13 games. The Heat were the first of five consecutive Eastern Conference powers the Mavs will face, with the Milwaukee Bucks, Boston Celtics, Philadelphia 76ers and Toronto Raptors on Dallas' schedule over the next eight days.
Dallas fell behind by as many as 24 points against the Heat before rallying but couldn't finish against a 19-7 Miami team.
"We have a lot of guys that can step up and play well," said Mavs power forward Kristaps Porzingis, who had 22 points, 14 rebounds and two blocks in the loss. "Even without having him in the second half, we made that run and we came back and fought. We were right there to win the game. We had so many opportunities to win the game. That's what frustrating. We've got to keep working."