Coach Brian Flores announced Wednesday that further testing revealed Tagovailoa suffered fractured ribs in Week 2's loss to Buffalo.
"We decided to run some more tests, we ran them, and we got this information," Flores said, via Jeff Darlington of ESPN. "It's unfortunate. I don't want to put a timetable on it. He's taking it day to day."
Flores stopped short of saying the Fins are considering placing Tua on injured reserve, noting that the QB is "week-to-week" with the rib fracture. The injury is not a season-ender.
The second-year QB suffered the injury just eight snaps into Sunday's blowout loss to the Bills. He completed 1 of 4 passes for 13 yards and took two sacks before exiting.
"I felt pretty bad," Dolphins right tackle Jesse Davis said Wednesday of Tagovailoa's injury. "Anytime you see your QB go down and it's your fault, it's a bad deal."
Initial exams provided optimism about Tagovailoa's injury, but the fracture was revealed after further testing.
"This is a tough kid," Flores added. "He wants to play. He's trying to play. But we're going to save him from himself."
Brissett will make his first start in a Dolphins uniform against a feisty Raiders defensive front.
The 28-year-old QB completed 24 of 40 attempts for 169 yards and one interception in relief during Sunday's 35-0 defeat. Despite not getting on the scoreboard, the Dolphins moved the ball for stretches, getting inside the Bills' 35-yard-line on four of Brissett's drives.
Brissett is 12-20 in his career as a starter (11-19 with Indianapolis and 1-1 with New England). His last start came in 2019 in Indy.
Reid Sinnett will serve as the backup Sunday.