Johnson said he suffered the injury in March while filming a video of one of his workout sessions for the team's website.
"It got to the point where, 'It's got to be something more than a strain or something' because every time I sit down it's hurting," Johnson told the newspaper. "Every time I stand up it's hurting. I lay down, it's hurting. I went through rehab and it wasn't helping.
"So I got the MRI, and it was a slight fracture."
Johnson told the newspaper he injured his back doing a power-rope exercise.
The receiver twisted his ankle when the Bills' voluntary workouts began in April. He told the newspaper that he was trying to test out his back and "ended up stepping on a cone."
Johnson, who has led Buffalo in receiving yards in each of the past three seasons, returned to participate in team drills last month.