Texans coach Gary Kubiak said Johnson would undergo tests Sunday night, so his status remained uncertain for next week's game against Oakland.
"Now, if we have to go without him for a period," Kubiak said, "we're going to have to really ask some guys to step up to the plate."
Texans owner Bob McNair refused to speculate on the severity of Johnson's injury.
"I am not going to be overly concerned until the MRI," McNair told ESPN.
The five-time Pro Bowler caught a 13-yard pass in the second quarter and was holding the back of his right leg near the knee after he fell to the ground. He pulled up at the end of the play, but was not hit.
Johnson remained on the field and was looked at by trainers before gingerly walking off on his own power.
He then walked to the locker room with members of the training staff.
"The way he went down was deflating," Kubiak said, "because it was like he just got shot. So that scares the heck out of you."
Houston (3-1) got starting running back Arian Foster back Sunday after he missed last week's game in Pittsburgh with a left hamstring injury. However, Foster's backup, Ben Tate, injured his groin against Pittsburgh.
Information from ESPN The Magazine's Ryan McGee and The Associated Press contributed to this report.