The Lions were on the verge of solidifying their hole in running back with the concussion injury to Jahvid Best by trading for the Philadelphia Eagles' Ronnie Brown.

Now they may have taken a step back.

According to multiple sources, the trade has been voided because Jerome Harrison, the running back traded to the Eagles in exchange for Brown, has previously undisclosed health issues.

Best, the Lions' starting back, suffered the concussion injury in Sunday's loss to the San Francisco 49ers. It is unknown whether Best will be able to play against Atlanta on Sunday or how long he will be out.

He had a hard time with some of the lights after the game, Lions coach Jim Schwartz said Wednesday. 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.

The Lions have only veteran running back Maurice Morris and second-year player Keiland Williams in their backfield if Best is unable to play. Detroit also signed running back James Davis to its practice squad Wednesday.

The Lions were supposed to get Brown from the Eagles in exchange for Harrison and an undisclosed 2013 draft pick. The team has struggled with the run all year, ranking 25th in the league. Best had suffered his second concussion in three months and had concussion issues during his junior year of college as well.

Brown, who played for current Lions offensive coordinator Scott Linehan during his rookie season for the Miami Dolphins in 2005, was a logical pickup and someone who would assure the Lions versatility and production. He was a Pro Bowl alternate after rushing for 916 yards and 10 touchdowns for the Dolphins in 2008 and has always been an above-average receiver, accumulating almost 1,500 yards in his six-year career.

Taking into consideration the Lions' weak running attack, receiving has been a critical attribute in the team's passing offense, and at 230 pounds, Brown would be a much better power option than Best, who is only 200 pounds. Also, the Lions would have the option to start using some of the Wildcat offense, which Brown was the key component of in the original scheme the Dolphins introduced in 2008.

The Eagles have traded for Harrison for the second consecutive season. Last year, Harrison was shipped from the Cleveland Browns in exchange for running back Mike Bell, and rushed for 239 yards on 40 carries in eight games with the Eagles.

Harrison was a free agent following the 2010 season and wound up in Detroit, where he rushed for 41 yards on 14 carries in three games played. When Harrison was originally set to be a restricted free agent, the Eagles showed their opinion of the running back by placing a second-round tender on him.

The Eagles, with Brown, have the number one rushing attack in the NFL, led by breakout star LeSean McCoy with Brown as backup.

The Lions, on the other hand, are trying to improve their running game and could have been a more threatening playoff contender with Brown in the backfield.