Jon Bostic did not sulk after veteran D.J. Williams beat him out for the starting middle linebacker position in the preseason. Instead, the second-round rookie embraced his special-teams role with an understanding his time to play defense would come.

It arrived Thursday night sooner than expected. Williams tore his left pectoralis muscle tendon during the 27-21 victory over the Giants at Soldier Field and will miss the rest of the season.

Now, Bostic must try to build on a promising preseason and help keep together an injury-riddled defense.

"It's the next-man-up mentality, and he's mentally ready," coach Marc Trestman said. "The experience he had will certainly help him."

Bostic played 21 defensive snaps Thursday, most of them after Williams got hurt late in the third quarter.

"I made a lot of mistakes out there," he said. "Really, I just have to get settled back in, calm down and play by our rules."

Bostic evaluated his mistakes with a sense of understanding. Although he wouldn't specify them, he emphasized the importance of learning from the experience.

Williams left the field favoring his left shoulder and chest area after wrapping up 6-foot-4, 264-pound running back Brandon Jacobs. He is the third defensive starter to suffer a season-ending injury in the last four games, joining defensive tackles Henry Melton and Nate Collins.

Williams, 31, in April signed a one-year free-agent contract worth $1.75 million.

He played mostly in the base defense, but he began participating in the nickel package some against the Saints last Sunday.

Williams politely declined an interview request in the locker room Thursday, citing the need to learn more about his injury.