J.J. Watt suffers serious knee injury against Chiefs

J.J. Watt left the Houston Texans' game with the Kansas City Chiefs midway through the first quarter with a knee injury on Sunday.

Watt went down after rushing Chiefs quarterback Alex Smith on a pass play despite there appearing to be almost no contact.

The 28-year-old had to be helped off the field, clearly favouring his left leg.

After being checked in the medical tent, he was transported on a cart to the locker room.

The Texans announced Watt was out with a knee injury.

It is tough luck for the Texans defensive end, who missed all but three games last season after undergoing two back surgeries.

The three-time NFL Defensive Player of the Year is still seeking his first sack of the season.