While the team has not yet commented on Mallett's injury, two sources told the Chronicle that his torn muscle will require surgery and will end his season.
Mallett sustained his initial injury during preparations for Houston's Nov. 16 road game against the Cleveland Browns, Mallett's first start.
Last week, ahead of the Texans' game against the Bengals, he took part in practice and was listed on the team's injury report as 'probable' with a chest injury.
He went 21-of-45 against Cincinnati, throwing for 189 yards and one interception, earning a quarterback rating of 49.2. He reportedly aggravated the injury during warm-ups.
In three games for Houston this year, Mallett has gone 41-of-75 for 400 yards, two touchdowns and two interceptions.
Ryan Fitzpatrick, who started the first nine games of the season for the Texans, is likely to replace Mallett in the lineup. Fitzpatrick went 4-5 as the starter, completing 61.8 percent of his passes for 1,960 yards, 11 touchdowns and eight interceptions. Tom Savage, a rookie, is the other quarterback on the Houston roster.
The Texans (5-6) play the Tennessee Titans at home on Sunday.