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Lions cornerback Rashean Mathis said the Chicago medical staff kept him from returning to Sunday's game against the Bears. (Associated Press)

DETROIT -- Cornerback Rashean Mathis passed the Detroit Lions' concussion tests and was cleared to return to Sunday's game against the Chicago Bears.

But the Bears overruled Detroit's doctors, which kept Mathis from returning.

Mathis suffered a possible concussion in the first quarter of Sunday's 40-32 win and underwent two tests. He said he passed one on the sideline, then again during halftime and was told he could return in the second half.

But Chicago's medical staff, which was first to respond to Mathis on the field, said they believed the safety lost consciousness after the play and wouldn't clear him.

Mathis said he does not believe he lost consciousness.

"I ran out on the sideline because I took another test at halftime. They're like, 'You're good,'" Mathis said in the locker room after the game. "So I ran down the steps, thought I was cleared to play -- but then our head guy said their doctor said, 'You might have went out for a second and protocol says you can't go.'"

Asked about a possible conflict of interest, with the Chicago staff keeping a Detroit player out of a battle for first place, Mathis said "that's not my job to get into that."

Mathis was injured while trying to tackle Chicago tailback Matt Forte. He crumpled to the field, but trotted off on his own power after being examined by both medical staffs. As part of his testing, he was given five words and asked to repeat them, and was given a six-number sequence that he had to repeat backwards.

"I'm going to let you take the test," Mathis told a reporter. "You repeat 'em backwards: 6-1-7-8-2-5."

The reporter failed the test.

"But I passed 'em -- twice," Mathis said. "And just a little light test. Balance too."

Mathis said he was frustrated not to play in the second half, even though he felt no ill effects and had been cleared by his own staff, but the win helped mitigate those concerns.

Detroit is 3-1 and moves into a first-place tie with the Bears (3-1) atop the NFC North.

"I feel good," he said, "but I feel even better being that we won. I probably would have been P'd off if we didn't pull out the victory and I didn't play."

Rookie Darius Slay filled in for Mathis in the final three quarters at right cornerback. He had started the first two games, but was benched for poor play in both and didn't see the field at all last week against Washington.

The only other known injury for Detroit occurred to the other starting corner, Chris Houston, who left with a leg injury late in the game. Detroit compensated by moving Slay from the right to left side and inserting Jonte Green at right cornerback.