Brown was signed to a one-year, $1 million contract as a free agent, but hasn't been a solid fit with the Eagles' offense.
In six games, he has rushed for only 38 yards on 13 carries with one lost fumble. He's averaging just 2.9 yards per carry.
Brown is a former Miami Dolphins starter and first-round draft pick from Auburn.
The Lions were looking for a running back due to Jahvid Best's concussion.
Harrison has previously played for the Cleveland Browns and the Eagles.
He has gained only 41 yards on 14 carries in four games this season for the Lions.
However, he did rush for 239 yards and one touchdown last season in Philadelphia. Harrison also caught eight passes for 43 yards.
For his career, he has rushed for 1,681 yards and seven touchdowns with 70 receptions for 590 yards and three scores.
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