Australian NBA star Andrew Bogut has left the court in agony with a broken leg, just moments into his debut with the Cleveland Cavaliers, after joining the team as a free agent.

Bogut entered the Cavs' game against Miami and went down with an injury to his left leg.

He was defending Heat forward Okaro White, whose leg collided with Bogut, as he attempted to dribble around the 32-year-old Aussie.

Bogut was heard to say "it's broken", as he was attended to by team-mates. The Cavaliers later confirmed his self-diagnosis.

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"Initial x-rays and evaluation ... revealed a fractured left tibia," a team statement said. "Bogut was taken to the Cleveland Clinic for further imaging and evaluation, and additional updates will be made as they become available."

Superstar team-mate LeBron James said he didn't need the confirmation.

"I heard it [the injury]. I heard it break, as soon as the collision happened, I heard it break.

"When I went over to him, he said 'I already knew, I heard it crack'," James said after a 106-98 defeat. "It's very deflating, it's a tough moment.

"We were all very excited about bringing him here and him getting some games under his belt before the play-offs. We will hope for the best, it's a tough one especially for him, first of all, and for the whole club."

Bogut was taken from the court to the locker room, having played just 58 seconds. He could not put any weight on his left leg, as he was assisted off by team-mates James Jones and Tristan Thompson.

The Australian centre had already registered two fouls, before leaving the court, having sustained the injury at the 11:38 mark of the second quarter.

After an injury-prone career, the Cavaliers would have hoped Bogut was not out of action long-term, having signed him to bolster their looming playoffs campaign.

Bogut only signed with Cleveland last week, turning down bigger contracts and more playing time to help the Cavs defend their NBA title.

Bogut received a huge ovation when he took the floor for the first time with 36 seconds left in the first quarter and another one second later when he picked up a foul with the kind of physical play the Cavs were planning to get from him.

The Cavs signed Bogut to a $385,000 contract for the rest of the season, adding him to beef up their front court that is missing All-Star forward Kevin Love, who is still weeks away from returning from knee surgery.

Bogut injured his left knee in Game Five of the NBA Finals last year for Golden State and hurt his right knee in December, while playing for Dallas. Before the game, Bogut was in a playful mood, as he discussed his choice to join the Cavs.

"It was a tough decision," Bogut said. "It came down to two teams - Boston and Cleveland - and I decided to go with Cleveland, obviously.

"It's the best opportunity to try to win a championship, playing with one of the best, if not the best, players to play the game.