A source with knowledge of the injury told ESPN that Mathis has a sprained MCL. The source believes it's a Grade 3 sprain (complete tear), which would sideline Mathis for approximately four to eight weeks. The injury, however, is still being evaluated.
Mathis wasn't the only Eagles starter on the offensive line to suffer an injury Sunday. Right tackle Allen Barbre, who is filling in for the suspended Lane Johnson, suffered a high ankle sprain, a source confirmed.
Both Mathis and Barbre are expected to see specialists later in the week to further evaluate their injuries.
The Eagles next play Monday night at the Colts.
The nature of both injuries was reported earlier Monday by the Philadelphia Inquirer.
The Eagles have nine offensive linemen on their 53-man roster, but only six are healthy. Guard/tackle Matt Tobin (ankle) was expected to return to practice this week but his status is still unclear.
Both Barbre and Mathis were injured in the first half as the Eagles went into halftime trailing 17-0 to the Jacksonville Jaguars. The Eagles posted a 34-point second-half comeback with backup center David Molk playing left guard, a position he said he hadn't practiced at all summer, and Andrew Gardner, who played mostly at left tackle during the preseason, filling in for Barbre at right tackle.
Quarterback Nick Foles was sacked five times in the first half, most of which was played with the starting line in front of him. In the second half, with Molk and Gardner filling in, Foles wasn't sacked at all.
Information from ESPN.com Eagles reporter Phil Sheridan is included in this report.