(TMZ) - WARNING ... this is one of the most disgusting sports injuries you'll ever see.

NBA star Gordon Hayward suffered a brutal lower leg injury just 5 minutes into his Boston Celtics debut against the Cleveland Cavs Tuesday night ... and it's absolutely cringeworthy.

Hayward went up for an alley-oop from new teammate Kyrie Irving when he landed hard on the court. He was immediately tended to by medical personnel and was taken away on a stretcher.

No official word on how long he'll be out ... but it doesn't look good.

Photo: Getty Images