Miller was the Texans' leading rusher (973 yards) en route to his first Pro Bowl in 2018.
With him unavailable, the spotlight turns to Duke Johnson Jr., whom Houston acquired from the Cleveland Browns in return for a 2020 fourth-round draft pick that can become a third-rounder, according to NFL Network's Tom Pelissero.
Johnson got crowded out of the Browns' backfield following the emergence of Nick Chubb and arrival of Kareem Hunt, who is suspended for the first eight games of the regular season.
The 25-year-old has shown promise in a limited role, though. In four seasons, he has 1,286 rushing yards on 4.3 yards per carry and five touchdowns. He has also caught 235 passes for 2,170 yards and eight touchdowns.
Even before Miller's injury, the Texans likely viewed Johnson as a key contributor on the ground and in the passing game. Now, he'll get the chance to prove he can be an every-down back in the NFL.
The bigger question is whom Houston will tab as the backup.
Undrafted free agents Karan Higdon (28 carries, 78 yards) and Damarea Crockett (17 carries, 83 yards and one touchdown) have gotten the most touches during the preseason. Houston's preseason finale Thursday against the Los Angeles Rams could shed some light on the team's backfield hierarchy.