Coach Pat Shurmur said Hillis felt ill when he arrived Friday morning at the team's facility.
"Whatever it is, we don't feel like it will affect him on Sunday," Shurmur told The Associated Press. "We felt the best course of action was to just send him home and give him the best opportunity to get as much rest and fluids as we could."
The Browns are in position to open the season 2-1 for the first time since Butch Davis coached them to a 9-7 record in 2002. Incidentally, that start led to the team's only playoff berth since the franchise rebooted in 1999.
Notes: Cleveland's Josh Cribbs is questionable with a groin injury. Offensive lineman Tony Pashos and wideout Mohamed Massaquoi are both questionable with ankle injuries. For the Dolphins, cornerback Vontae Davis has been ruled with a hamstring injury. Safety Chris Clemons and nickel back Will Allen are questionable with hamstring injuries. Defensive end Tony McDaniel is out with a broken right hand.