On Wednesday, Ian Rapoport and Tom Pelissero of NFL Network reported he is out "at least a week or two" and is preparing to miss as many as three to six weeks.
Jeff Zrebiec of the Baltimore Sun first reported the injury but said that back soreness may limit his repetitions or cause him to miss a few practices.
Flacco nursed a minor shoulder injury in 2016, but the ailment didn't force him to miss any games a season after he tore his ACL.
Other than in 2015, the 32-year-old has played all 16 games in his eight other seasons in the NFL.
So long as Flacco is sidelined, backup Ryan Mallett should see the bulk of the first-team snaps in practice.
Mallett's last meaningful action came in 2015, when he completed 59.8 percent of his passes for 566 yards, two touchdowns and two interceptions in two starts for the Ravens. Dustin Vaughan is the only other quarterback on the roster.