Watson tore the ACL in his right knee, a source told ESPN's Chris Mortensen, confirming a Houston Chronicle report.
The injury occurred about halfway through practice on a grass field; Watson appeared to trip or fall but did not yell, someone present at practice told Schefter. Watson got up and felt his knee before it was looked at, but the person said no one believed the injury was season-ending.
Watson did not finish practice. Tom Savage took over at quarterback.
The Texans will sign Matt McGloin to back up Savage, a source told ESPN's Dan Graziano, confirming a Houston Chronicle report. Also, in a text to ESPN's Jeff Dickerson, quarterback TJ Yates said he's signing with the Texans in backup role.
NFL Network first reported the possibility that Watson's ACL was torn and that it was a noncontact injury.
During his freshman season at Clemson in 2014, Watson played with a torn ACL in his left knee, coach Dabo Swinney said at the time. Watson suffered the injury in practice and played the following week. Watson was told he could continue playing with a brace securing the knee. Swinney released a statement on Twitter:
The loss of Watson is a blow for the Texans (3-4). His Total QBR of 81.9 leads the NFL and is the highest for any player in his first seven career games over the past 10 seasons, according to ESPN Stats & Information research. Watson also ranks first in rushing yards among quarterbacks (269) and is tied for first in touchdown passes (19).
Watson has helped make the Texans the highest-scoring team in the NFL, with 30.7 points per game. Last season, Houston ranked 28th in points.
Watson's 19 touchdown passes are four more than the Texans quarterbacks (Brock Osweiler and Savage) combined for all of last season.
"It's just unfortunate. It's terrible news," cornerback Richard Sherman, whose Seattle Seahawks edged the Texans in a thriller last weekend, said when he heard the news. "Feel terrible for him and his family. He had such a bright future. He still has a bright future, but his rookie year is cut short. He probably had one of the best rookie months that anybody's had in a long time, so I'm sure he'll still be in the running for rookie of the year, but it's just not anything you want to hear.
"It just sucks that he doesn't get a chance to continue his season and finish it out. It's just a freak accident, man. But it sucks."
"Lil bruh I know at this time things may be scary. Your first year in the league; running for MVP; trying to win a championship, trying to get the spot you deserve, to defy the odds, to do the things they said you can't, and it all gets taken away.
"Look back in life to when you faced adversity and thought you would not overcome it, and then you did. As life went on, more and more obstacles came your way that you had to again overcome time and time again but you got through it somehow. Now life throws you bigger problems and again you're faced with a test.
"Everything is a blessing in disguise. Truly believe that this all happens for a reason. Take this time to evaluate and appreciate everything you've been blessed with. Life's still good, kid. WE LIVINN!! And we will be back better than ever. I'll see you this off-season lil b."