NFL Network's Ian Rapoport reported the Saints placed Brees on injured reserve, meaning he will miss at least three games.
Brees has had significant injuries in each of the past two seasons. He missed five games in 2019 with a torn ligament in his right thumb but still threw for 2,979 yards with 27 touchdowns and an NFL-high 74.3 completion percentage.
In the second quarter of New Orleans' 27-13 win over the San Francisco 49ers in Week 10, he landed awkwardly after being hit by Kentavius Street. He finished the quarter but was replaced by Jameis Winston in the second half.
Per Rapoport, Brees was expected to miss at least two weeks after being diagnosed with cracked ribs and a punctured lung.
The 41-year-old remains effective when healthy. He leads the league with a 73.5 completion percentage and has thrown for 2,196 yards with 18 touchdowns through nine games.