Johnson fell awkwardly running a route on Sunday morning and had an MRI exam later in the day that showed a "mild strain."
"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."
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."
Coach Gary Kubiak said Johnson wanted to continue practicing, but Kubiak took the cautious route. Johnson watched the rest of the outdoor half of the workout with a towel wrapped around his head then went to Reliant Stadium when the practice moved indoors.
"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."
Johnson sat out three to four weeks after his knee surgery in May, and he turned 31 in July. After playing in all 16 games in 2008 and '09, Johnson sat out three games late in 2010 with a sprained right ankle. Last year, he hurt his right hamstring against Pittsburgh in October then hurt his left hamstring against Atlanta in December.
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."
Basketball Hall of Fame center David Robinson attended Sunday morning's session as a guest of Texans running backs coach Chick Harris. Robinson chatted with LB Brian Cushing and Kubiak and posed for pictures with several players. "Well, I'm trying to figure out what team I want to root for because my wife (Valerie) is from Chicago, so she's a Bears fan," Robinson said. "So every year I've been picking different teams, but this year I'm going to be a Texans (fan). I have a (David) Carr jersey at home from a couple years ago but now I'm going to have to get another jersey and be a Texans fan." ... Kubiak said CB Kareem Jackson hurt his left hamstring on Sunday and will probably 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. He backed up DeMeco Ryans last season before hurting his quad and getting carted off the field in an October game against Jacksonville. "It's kind of been one thing after another for him, I feel really bad for him," Kubiak said. "He's worked really hard."