Watt has been out since tearing a pectoral muscle Oct. 27.
"When I got hurt, I just assumed I was done [for the season]," Watt said after returning to practice Dec. 24. "I didn't even know there was a possibility. And then literally a couple days after the surgery, I was feeling so good that ... I just started asking questions. I said, 'Is it remotely possible?'
"We left the door open for a possibility. And from there, it was just literally, day by day, just ... doing whatever I can do. And we arrived here."
To make room for Watt on the roster, the Texans have placed starting safety Tashaun Gipson on injured reserve.
Watt said he understands there is an increased risk of tearing the muscle again, but it is worth it to have the chance to play again this season. He played in all 16 regular-season games in 2018, finishing with 16 sacks, which ranked second in the NFL.
In addition, Texans wide receiver Will Fuller V is expected to be available against the Bills, a team source told ESPN's Stefano Fusaro.
Fuller, who was limited in Tuesday's practice, has been dealing with a groin injury he suffered Dec. 21 against the Tampa Bay Buccaneers.
Officially, coach Bill O'Brien said Fuller is "progressing" and "heading in a positive direction to be available," but he did not commit to Fuller playing Saturday.
The Texans could use Fuller's speed against the Bills' secondary, combining him with wide receiver DeAndre Hopkins, who will return after missing Sunday's game against the Tennessee Titans with an illness.
The Texans host the Bills at 4:35 p.m. ET Saturday (ABC/ESPN).