Johnson fell awkwardly while running a route Sunday morning and had an MRI exam later in the day that showed a "mild strain." A team source told NFL.com and NFL Network's Charley Casserly that Johnson would miss "a few practices."
"It just tightened up on me," the five-time Pro Bowl receiver said. "I had hit the ground pretty hard, stretching out for a ball. That was pretty much it."
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Johnson missed nine regular-season games last season with hamstring injuries and was sidelined from the first organized team activities in the spring after arthroscopic surgery on his left knee.
"Nothing that I'm not used to, doing rehab," Johnson said. "It's very frustrating, but it happens, man. I wish I had some control over it. Unfortunately, I don't."
"I felt pretty good, because he wanted to go back out there," Kubiak said Sunday afternoon. "When a guy is telling you that, I don't think it's too bad."
On Sunday afternoon, Johnson wasn't even walking with a limp. He said he'll know "in a few days" when he'll be able to return to practice.
"Everybody's just saying, 'Just be smart about it,' " Johnson said. "You don't want to rush in, come back out here and have something happen again. We'll be smart about it, and see what happens."
Notes: Kubiak said CB Kareem Jackson hurt his left hamstring Sunday and probably will skip Monday's workouts. "Just a little sore," Jackson said, "not a pull, not a tear or anything. Nothing serious." ... Kubiak said LB Darryl Sharpton had a setback in his recovery from a right torn quadriceps injury. "We're trying to get him to the point where he's working with the team," Kubiak said. "We turned him loose with the team, and he kind of had a setback." Sharpton went on injured reserve late in 2010 with a shoulder injury.
The Associated Press contributed to this report.