Atogwe, a five-year veteran, was hurt on the final play of the first half in Sunday's loss at Chicago when he was hit by Matt Forte's helmet while assisting on a tackle. He has 84 tackles, two interceptions, three forced fumbles and two fumble recoveries and had started 60 consecutive games dating to the 2006 opener.
Rookie offensive tackle Jason Smith met with an independent consultant Tuesday and underwent an MRI exam that showed no problems beyond a concussion that has sidelined him two games. Smith has failed three tests that showed he's still experiencing symptoms, the latest on Monday.
Coach Steve Spagnuolo said the MRI exam was taken as a precaution with Smith, the second overall pick of the draft.
"It was just more of a reassurance thing," Spagnuolo said.
Atogwe held out hope on Monday that he could finish the season wearing a brace, but the team decided against it. Coach Steve Spagnuolo said the risk was too great for an injury that will require surgery.
"I really couldn't probably sleep or live with myself if O.J. forced himself in there and all of a sudden that thing popped out again," Spagnuolo said. "There was a chance that was going to happen. For his health and his career, I think it's best."
The Rams also signed rookie quarterback Mike Reilly from the Green Bay Packers' practice squad, making him the third-stringer. They also signed free safety Clinton Hart and released linebacker Dominic Douglas.
Both of the backup quarterbacks behind Kyle Boller played in NCAA Division II and faced each other in the playoffs last November. Keith Null was a sixth-round pick out of West Texas A&M and Reilly was undrafted after playing at Central Washington.
"It's kind of few and far between that you run into somebody that played at Division II, let alone somebody you played against," Reilly said. "I think no matter where you are, you've got to prepare as if you're going to play."
Spagnuolo said the signing had no relation to starting quarterback Marc Bulger's recovery from a fractured left shin bone. Bulger has missed two games, was still on crutches earlier in the week and was scheduled for a follow-up MRI exam.
"Oh yeah, I was shocked," Hart said. "Normally, they don't cut the starter. But this is an opportunity."