CLEVELAND – Boston forward Gordon Hayward sustained a fractured left ankle on an alley-oop attempt in the first quarter of Tuesday's NBA season opener at Cleveland, the Celtics confirmed in a release.

The horrific injury, which occurred at the 6:45 mark of the first quarter, looked severe with Hayward’s leg and ankle turned in an unnatural position. Hayward’s leg was immobilized, and he was carted off on a stretcher.

Players immediately called for medical attention once they saw Hayward’s leg.

Watch: Hayward fractures ankle (Warning: Disturbing)

Hayward, who signed a four-year, $128 million contract in the summer and joined his former college coach Brad Stevens in Boston, had two points before the injury.

Celtics players huddled on the sidelines, and concerned silence filled Quicken Loans Arena.

Cleveland’s LeBron James pulled his jersey over his face when he saw Hayward’s leg and shook Hayward’s hand as medics wheeled the Celtics star off the court.

The injury is devastating for the Celtics, who signed Hayward and acquired Kyrie Irving to challenge the Cleveland Cavaliers and win the Eastern Conference championship.

With the Utah Jazz last season, Hayward averaged career-high in points (21.9) and field-goal percentage (47.1%). He also averaged 5.4 rebounds and 3.5 assists and shot 39.8% on three-pointers and made his first All-Star team.

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