Thomas is rehabbing from a torn Achilles suffered in Week 16. The Texans acquired him from the Denver Broncos before the 2018 trade deadline in exchange for a fourth-round pick and a swap of seventh-rounders.
The five-time Pro Bowler was due $14 million on his base salary in 2019.
Thomas said he was "not thinking about retirement" in January while rehabbing but acknowledged his future in Houston was dim.
The 31-year-old had 59 catches, 677 receiving yards, and five touchdowns in 2018. He recorded five consecutive 1,000-yard campaigns from 2012-16 during his eight-and-a-half seasons with the Broncos.