According to Houston's official injury report, Cobb is dealing with a toe injury, while Stills has a quadricep injury.
Stills was a late addition to Houston's roster last season when he came over on Aug. 31 as part of the trade with the Miami Dolphins for Laremy Tunsil.
Injuries impacted Stills in 2019 when he sat out two games in October with a hamstring issue. He's fallen off the radar this season with just 144 yards on 11 receptions.
Fantasy managers who even had Stills on their roster at this point probably aren't faring well in their league.
Cobb has been productive in this offense through 10 games. The 30-year-old ranks third on the team with 38 receptions, 441 yards and is tied for second with three touchdowns. His fantasy role is as a low-tier flex option.
The absence of Stills and, especially, Cobb should open things up for Will Fuller and Brandin Cooks. Fuller has taken off as Houston's go-to wideout with 708 yards, six touchdowns and 15.1 yards per reception. He's going to be Deshaun Watson's main target against a Lions defense that's tied for 15th in fantasy points allowed to opposing receivers.