The defensive end was injured while being blocked midway through the first quarter and fell to the ground.
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."
"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.
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.
Update: DE J.J. Watt has suffered a tibial plateau fracture to his left leg.— Houston Texans (@HoustonTexans) October 9, 2017
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