His agent, Jack Mills, told The Boston Herald and The Boston Globe that Johnson left the team Monday night after Coach Bill Belichick confirmed the seven-year veteran would be listed as inactive for Monday night's season opener against Pittsburgh.
The Patriots sent Johnson a formal five-day letter on Tuesday, giving him until Sunday to return or face a suspension and risk losing part of his salary.
"It's a business and options are limited, so here I am," Johnson said upon his return.
The reasons for Johnson's absence likely were related to playing time, or lack of such, or money. The recovery of counterpart Tedy Bruschi from a knee injury could limit Johnson's on-field time, or he might be dissatisfied with having restructured his contract six times in four years.
"I'm not really going to get into my feelings on it or any of my conversations on it with Ted," Belichick said. It's not the first time a player has left the Patriots unhappy."
Belichick likely was referring to wide receiver Terry Glenn, who had a history of doing it, often resulting in team suspensions, before he was traded to Green Bay.
Belichick said he respects Johnson as a player and person, but notes how consistent he has been in handling such situations.
"Without getting into detail about them, whether they're personal or, in some cases, things that I'm not really able to publicly discuss, they're consistent," he added.
Johnson, a second-round draft pick in 1995, had a career-high 142 tackles in 1998, but has been plagued by injuries, including a pair of torn pectoral muscles. Also, a concussion forced him to miss two games this preseason.