Suddenly, the Detroit Lions have major issues at running back.
Two league sources told ESPN NFL Insider Adam Schefter that the trade was voided because Harrison has previously undisclosed health issues.
Brown arrived in Detroit but never got to practice.
Lions spokesman Bill Keenist said the team would have no further comment.
The Eagles announced Tuesday they dealt Brown to Detroit for Harrison and an undisclosed pick in the 2013 draft. Detroit wouldn't confirm the move because the franchise doesn't until deals are done.
The Lions were looking forward to adding Brown to a backfield that might be without starting running back Jahvid Best for a while. He had his third concussion Sunday in a loss to San Francisco.
Best walked across Detroit's locker room, trying to avoid eye contact and conversation.
The running back did slow down slightly, but wouldn't or couldn't answer whether he'll play Sunday against Atlanta.
"I have no idea," Best said Wednesday after missing practice because of the concussion.
Unfortunately for Best and the Lions, timetables are elusive when it comes to concussions, and he says this one is his third. He had one in the second preseason game this year and another in 2009, his final season at California when a spectacular fall sent him to the hospital with a concussion and sore back.
Best was knocked out of Sunday's loss against San Francisco.
"He had a hard time with some of the lights after the game," coach Jim Schwartz said. "Those are signs that you've got to be concerned about and you have to be objective about. We'll have a whole protocol that he has to go through to get back on the field.
"Coming back from a concussion isn't like coming back from a sprained ankle or a bad shoulder or a pulled hamstring. Some of those things you can tough out. You can't tough out a concussion."
With Best out, the Lions (5-1) sounded as if they were ready to plug Brown into the lineup Sunday against the Falcons (3-3) despite not being able to put him on the practice field Wednesday.
"Maurice has answered the bell any time its been run for him," Schwartz said. "Keiland has shown some really nice things."
Detroit's depth in the backfield took a hit during the preseason when rookie running back Mikel Leshoure had a season-ending injury. The team signed running back James Davis to the practice team, released tight end Nathan Overbay from the practice squad, and Davis might have a chance to quickly get activated to give Detroit a third running back against Atlanta.
Best, in his second season out of Cal, has played in all six games this season, rushing 84 times for 390 yards at 4.6 yards per carry and two touchdowns without a fumble. He also has 27 catches for 287 yards and a score.
Information from The Associated Press contributed to this report.