The defensive end was injured while being blocked midway through the first quarter and fell to the ground.
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Fans described seeing Watt hurt on the field as "scary."
"I love J.J., he's the heart and soul of our team," a fan said. "We have to keep going."
Fans react to J.J. Watt injury
"We got to work as a team together," another fan told ABC13.
On Twitter, Watt's younger brother, T.J., also posted a message.
"Heartbroken for you brother," T.J. Watt tweeted.
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The three-time Defensive Player of the Year returned this season after missing the last 13 games of last season after his second back surgery.
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