Harold recorded 34 combined tackles and two sacks for the Niners in 2017.
San Francisco selected Harold in the third round of the 2015 NFL draft after he enjoyed three solid seasons at Virginia. He left the Cavaliers having racked up 36.5 tackles for loss and 17.5 sacks.
As evidenced by the relatively small return the 49ers are receiving, Harold hasn't made much of an impact in the Bay Area. Through his first two seasons, he had just three sacks.
For this past season, Harold sat 41st in Bleacher Report's NFL1000 year-end rankings of the top 4-3 outside linebackers.
"On the edge, Harold did not provide the juice as a pass-rusher his build and volume would suggest, but he did maintain control of the perimeter well versus the run," NFL1000 scout Derrik Klassen wrote. "Harold was a trickier fit as an off-ball linebacker, often showing the athleticism to succeed, but not the savvy or awareness."
The Athletic's Matt Barrows noted what Harold's departure will mean for San Francisco's linebacking corps:
Even though Harold has struggled so far in the NFL, taking a flier on a young pass-rusher makes sense for the Lions. Detroit finished 20th in sacks (35) last season, with Ezekiel Ansah accounting for more than a third of that total (12).
Harold is likely to provide depth at outside linebacker behind Devon Kennard, who signed a three-year, $17.25 million contract with the team in the offseason.