Yahoo's Brian McIntyre first reported the new deal, which drops Johnson's 2013 base salary to $715,000 and converts the remaining $4.285 million into a signing bonus prorated over the next five seasons.
Defensive tackle Ndamukong Suh also agreed to a restructuring last week that saved $7.7 million in cap room.
The Lions have been active in free agency, signing running back Reggie Bush, safety Glover Quin and defensive lineman Jason Jones to big deals and retaining corner Chris Houston. General manager Martin Mayhew said he's trying to re-sign kicker Jason Hanson but said not to expect any more high-profile signings.
Detroit wants to extend quarterback Matthew Stafford's contract, which has two seasons left.
Johnson, 27, was the biggest bright spot in a 4-12 season, catching 122 passes for 1,964 yards and five touchdowns.
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