Detroit lost Steve Longa, a backup linebacker and key player on special teams, last week with a season-ending knee injury.
Harold started 24 games for San Francisco and played in 48 games over three seasons. He played in every game last season, starting 10 times, and had 34 tackles and two sacks.
San Francisco drafted the former Virginia standout in the third round in 2015. He has 84 tackles and five sacks in his career.
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