The 27-year-old pulled up with the strain in a training camp session on Friday and could miss the majority of his side's pre-season games.
Johnson told NFL.com: "It's just a process. It's gonna take time and patience. There's a lot involved in these hamstrings.
"I don't want to come back too soon and have to deal with it for the season, so I'm working with the guys in the training room and they're getting me right."
Johnson has had 1000 yard receiving seasons in his last three years in the NFL.