ORCHARD PARK, N.Y. -- Sam Cowart will be out indefinitely with an Achilles' tendon injury, leaving the Buffalo Bills without the player who has led them in tackles the last two seasons.

The middle linebacker will not need surgery on his partially torn right Achilles' tendon. But coach Gregg Williams said Monday he will miss "a significant amount of time" and it's unclear if he will return before the end of the season.

Cowart underwent an MRI earlier in the day.

The Bills were relying on Cowart in their new 4-3 defense. The fourth-year player was the squad's most experienced starter behind defensive end Phil Hansen.

"I don't know that it's any different whether Sam was here or not," Williams said. "Understand that there are still other leaders in that room."

Cowart was hurt while dropping back in pass coverage during the second series of Sunday's 24-10 season-opening loss to New Orleans.

Williams recalled Cowart saying on the sideline that it felt as if someone had kicked him. Having watched the tape, Williams said no one was around Cowart when he was hurt.

Cowart is coming off his first Pro Bowl season. He finished with 181 tackles despite missing the final four games of the season with an ankle injury.