Watson has been the favorite to win Rookie of the Year since taking over as the team's quarterback halfway through the season opener. Subsequent reports said an MRI confirmed the injury.
One of the best seasons for a rookie quarterback in recent memory is likely derailed because of a serious knee injury suffered in practice on Wednesday.
The injury is a brutal blow for the Texans, who have been lifted by Watson's play through eight weeks.
According to NFL Network's Ian Rapoport, Watson suffered what the Texans fear is a torn ACL in a non-contact drill. He faces an 8-9 month rehabilitation and should be back in time for the start of the 2018 season. He is tied for the National Football League lead in passing touchdowns (19) with the Philadelphia Eagles' Carson Wentz, while Houston leads the National Football League in scoring with 30.7 points per game. McGloin played for Houston coach Bill O'Brien at Penn State.
Watson was coming off the most impressive performance of his young career before suffering the injury.
The injury is awful news for the Texans, whose offense was thriving with Watson at quarterback.
The Georgia native was also named the NFL Offensive Rookie of the Month for October.
Watson completed 77 of 124 passes for 1,171 yards with 16 touchdowns and five interceptions in four games last month.