Indianapolis Colts quarterback Peyton Manning (18), shown in a playoff game in January, underwent a medical procedure on his left knee and will miss nearly all of preseason training, the Colts said. (UPI Photo/John Sommers II)

Manning had an infected bursa sac removed his left knee in an operation Monday. The team said the quarterback had been receiving "conservative treatment" for the inflammation since February.

When Manning reported an increase in pain and showed signed of infection, it was decided to remove the sac.

Colts team President Bill Polin issued a statement saying Manning would be out 4-6 weeks. If he was out six weeks, Manning would return Aug. 26, two days before the Colts' final exhibition game and 10 days before the regular-season opener against Chicago.

Manning has never missed a regular-season start in 10 seasons in the NFL. His 94.7 passer rating is second all-time behind Steve Young (96.8). He is an eight-time Pro Bowl selection and has passed for 41,626 yards and 306 touchdowns in his career.