San Diego Chargers general manager Tom Telesco says 33-year-old outside linebacker Dwight Freeney has a torn quadriceps muscle.
Telesco says the team doesn't yet know the severity of the injury or the time frame for recovery.
Freeney was hurt in the second quarter of San Diego's 30-21 victory against Dallas on Sunday.
Although he had only one-half of a sack, Freeney had applied constant pressure on the opposing quarterback.
After starring for Indianapolis for 11 seasons, Freeney was released in the offseason and signed with the Chargers in May after Melvin Ingram injured his left knee.
Freeney was Indianapolis' career sacks leader with 107 ½.
AP NFL website: www.pro32.ap.org