The 41-year-old fractured his ribs in back-to-back games and suffered a collapsed lung at the same time. He was injured in the wins against the Tampa Bay Buccaneers in Week 9 and the San Francisco 49ers in Week 10. He left the latter game early and did not return for the second half.
According to ESPN's Ed Werder (h/t ESPN's Mike Triplett), a second opinion confirmed that Brees has a collapsed lung and a total of five fractured ribs. Doctors have reportedly not established a timeline for the signal-caller's return.
ESPN's Dan Graziano reported shortly after the injuries came to light that Brees would miss at least two or three games.
The 13-time Pro Bowler leads the league with a 73.5 completion percentage, an area where he has paced other quarterbacks for the past three seasons. He exited Week 10 with 2,196 passing yards, 18 touchdowns and three interceptions on the season for the 7-2 Saints.
With Brees out, the Saints will turn to Jameis Winston, who joined New Orleans this offseason after five years in Tampa Bay. He has appeared in two games this season, completing seven of 11 passes for 75 yards and no touchdowns.
Winston, who was the No. 1 overall pick in the 2015 NFL draft, led the NFL last season with 5,109 passing yards and also threw 33 touchdowns passes, but he was also intercepted a league-high 30 times, which prompted Tampa to move on and sign Tom Brady.
Head coach Sean Payton will go with Winston under center in Week 11 against the Atlanta Falcons, and Rapoport's report suggests that New Orleans will need Winston to start again in Week 12 against the Denver Broncos.
The Saints also have utility player Taysom Hill, who plays essentially every skill position on offense, including quarterback.
Winston will be the primary quarterback until Brees returns, but there is little doubt that Payton will create plenty of opportunities for Hill to make plays from the quarterback position during that time as well.