Updated Friday, Oct. 11 at 2:35 p.m. CT

By Adam Hoge-

HALAS HALL (CBS) — The Bears have lost another key defensive player for the season.

Bears middle linebacker D.J. Williams is out for the rest of the season with a ruptured pectoral tendon, the team confirmed.

Bears head coach Marc Trestman admitted earlier Friday that he feared Williams had suffered a torn pec in Thursday’s 27-21 win over the Giants.

“There’s a certainly a chance (it’s torn),” Trestman said before it was confirmed.

Trestman added that he wasn’t sure how or when the injury occurred.

With Williams out, rookie Jon Bostic will arrive at Halas Hall on Monday as the team’s No. 1 middle linebacker.

“He’s going to get a lot of work on Monday and during the week,” Trestman said. “It’s a next-man-up mentality and I think he’s mentally ready to take on playing that position and the experience that he had will certainly help him down the road.”

Bostic spent the entire preseason as the No. 1 middle linebacker as Williams battled a calf injury, but Thursday night was the first time he saw any snaps on defense in a regular season game.

Williams is the fourth significant season-ending injury for the Bears’ defense. Cornerback Kelvin Hayden was lost with a torn hamstring in training camp, while defensive tackles Henry Melton and Nate Collins were both lost to torn ACLs in the last month.

Adam Hoge covers the Bears for CBSChicago.com and is a frequent contributor to 670 The Score. Follow him on Twitter at @AdamHoge.

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