Broncos head coach Vic Fangio told reporters Flacco would not play on Sunday against the Cleveland Browns after being diagnosed with a herniated disk in his neck. Ian Rapoport of NFL Network reported it's "likely" backup Brandon Allen gets multiple starts in the wake of the injury.
Adam Schefter of ESPN added that injured reserve is a possibility for Flacco when he's reevaluated after the Broncos' Week 10 bye.
Flacco threw for 174 yards on 20-of-32 passing with no touchdowns or interceptions on Sunday as the Colts rallied to win 15-13.
Since being drafted in the first round in 2008, the Delaware product has turned into one of the most durable quarterbacks in the NFL. The 34-year-old did not miss a game until his eighth season in the league, when a torn ACL limited him to just 10 games in 2015.
The Super Bowl XLVII MVP would not miss another game until last year when a hip injury snapped a string of 41 consecutive starts.
That ailment would ultimately lead to the end of his Ravens career.
Flacco completed 61.2 percent of his passes for 2,465 yards, 12 touchdowns and six interceptions in nine appearances for Baltimore last season but went just 4-5 as starter.
With Flacco sidelined, rookie Lamar Jackson stepped in and turned the team's season around, going 6-1 down the stretch en route to winning the AFC North. Flacco became expendable, and the Broncos opted to take a flier on the 34-year-old.
In his first year in Denver, Flacco has completed 65.3 percent of his passes for 1,822 yards, six touchdowns and five interceptions. He has guided the team to a 2-6 record.
While the Broncos committed to Flacco being the starter in 2019, it also used a second-round pick on former Missouri star Drew Lock, who remains on injured reserve with a thumb injury he suffered in the preseason.
After announcing Flacco's injury, Fangio said he hadn't decided on a the backup quarterback for Sunday's matchup with the Browns. Denver could activate Lock or promote Brett Rypien from the practice squad, per Darin Gantt of Pro Football Talk.