Doctors told Flacco and the team that a week of rest can calm down the back issue. Flacco hurt it while lifting weights and then underwent an MRI, a source told ESPN's Adam Schefter.
Injuries have been a major storyline for the Ravens, who open training camp Thursday morning. Running back Kenneth Dixon underwent season-ending knee surgery on Tuesday, tight end Dennis Pitta suffered a likely career-ending hip injury during offseason workouts and cornerback Tavon Young tore his ACL during an offseason practice.
Flacco, who is entering his 10th training camp, is known for his durability. His streak of 122 straight regular-season games played -- the fifth longest among quarterbacks in NFL history -- ended in 2015 when he tore two knee ligaments. Flacco has missed only six games in his career.
It was five years ago when Flacco led the Ravens to a Super Bowl title. He threw 11 touchdowns and no interceptions in a postseason capped by him being named Super Bowl MVP.
Since hoisting the Vince Lombardi Trophy, Flacco is 29-29 with an 82.5 passer rating. His 61 interceptions over that span rank as the third most in the league.
Flacco signed a three-year, $66.4 million extension in March 2016. His $24.55 salary-cap number is the highest of any player in the NFL this season.
The Baltimore Sun first reported news of Flacco's back injury.