Chicago Bulls forward Nikola Mirotic broke a bone in his face after he was punched by teammate Bobby Portis in a heated training session before the start of the NBA season.

Mirotic was hospitalised after the exchange and is out indefinitely with the Bulls to open their season Friday against the Toronto Raptors.

The two players shoved each other before Portis hit Mirotic and knocked him to the floor according to NBA website The Vertical.

The Bulls are in rebuild mode this season and both players were in the running to make their starting line-ups.

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The NBA season begins this afternoon with LeBron James and the Cleveland Cavaliers facing former teammate Kyrie Irving for the first time after his off-season trade to the Boston Celtics.