Johnson was injured while making a 21-yard catch just before the two-minute warning in the second half against the Tennessee Titans on Sunday. Cornerback Jason McCourty was covering him and safety Bernard Pollard joined the tackle, hitting Johnson in the head area.
Johnson's head also hit the ground hard as he fell.
The play was initially called incomplete, but was reversed after a review made clear that Johnson got both feet inbounds and maintained possession of the ball when he hit the ground at the Titans 2.
"He's being evaluated as we speak," Texans coach Gary Kubiak said during his postgame news conference. "... I have no diagnosis yet."
Johnson finished the game with eight catches for 76 yards. His final catch set up the Texans' game-tying touchdown in their eventual 30-24 overtime win.
"I talked to him and he said he was all right," tight end Owen Daniels said of Johnson. "He wanted to go back in, but protocol is that you can't at that point. I have never seen a more blatant blow to the head. A big miss by the officials, especially for a guy like Andre. I know you have to protect every player, but especially someone like him."