Mike Reiss breaks down how Rex Burkhead and Ja'Whaun Bentley being placed on IR affects the Patriots. (1:10)

FOXBOROUGH, Mass. -- The New England Patriots have placed running back Rex Burkhead and linebacker Ja'Whaun Bentley on injured reserve, the team announced Wednesday.

Also, the Patriots have signed free-agent linebacker/defensive end John Simon, formerly of the Indianapolis Colts.

Burkhead, who was on the injury report with a concussion after a season-opening win against the Houston Texans, left Sunday night's loss against the Detroit Lions in the third quarter. The team reported he had a neck injury.

Burkhead played seven snaps in the game, and it was unclear when the injury occurred. His final play was on the kickoff return unit with 3:45 remaining in the third quarter, when Lions safety Charles Washington collided with him, and Burkhead's head appeared to be thrust backward for a moment.

Burkhead, who was a free agent, had signed a three-year, $9.75 million contract to return to the Patriots in the offseason.

Without Burkhead, first-round draft choice Sony Michel becomes the top option at running back, with James White filling the "passing back" role and Kenjon Barner -- who agreed to terms Tuesday to return to the team -- adding depth.

Burkhead had 86 yards on 24 carries and three catches for 31 yards in the first three games of the season. He had played 59 of 184 offensive snaps over three games, with offensive coordinator Josh McDaniels pairing him up with White in a two-running-back package at times to account for a shortage at wide receiver.

Burkhead has also been part of the punt protection/coverage unit, along with the kickoff return and kickoff coverage units, so his absence will have a trickle-down effect on special teams.

Bentley, a fifth-round draft choice from Purdue, had made the largest impact among the team's rookie class. The trust he earned from the coaching staff was reflected in him having the communication device in his helmet Sunday against the Lions, when he led the defensive huddle. He also had an interception in the game.

Bentley's final play Sunday came with 4:52 remaining in the fourth quarter, as he was charging to tackle running back Kerryon Johnson and guard T.J. Lang landed on top of him. Bentley stayed down for a moment, seemingly due to the force of the 315-pound Lang.

Bentley's 13 tackles are tied for fifth on the team, as he had been on the field for 63.3 percent of the defensive snaps while also carving out a role on the punt coverage/projection and kickoff return units.

His absence projects to thrust third-year player Elandon Roberts into a larger defensive role alongside Dont'a Hightower and Kyle Van Noy, although special-teamer Nicholas Grigsby might also vie for playing time.