Johnson, listed first on the depth chart at his position, was injured on a running play Tuesday. He dropped immediately to ground and rolled over on his back before medical personnel arrived within one minute and put an air cast on his left leg.
“I don’t know the extent of the injury, but when we have concrete information we will get it out,” Atlanta coach Mike Smith said after practice. “Right now, we can’t speculate. We hope Mike is going to be fine.”
The Falcons and Cincinnati Bengals were practicing together for the second straight day.
Both teams will be missing some starters when Atlanta hosts Cincinnati in Thursday’s preseason opener.
Falcons receiver Julio Jones has sore hamstrings and linebacker Sean Weatherspoon is sidelined indefinitely with a dislocated left finger that resulted in an open wound.
Jones was limited in practice, but Weatherspoon stayed inside to get treatment.
Receiver A.J. Green has a bruised knee and left tackle Andrew Whitworth a sore knee following arthroscopic surgery for Cincinnati. Reserve receiver Andrew Hawkins is out, too, with a sore ankle.
Smith and his staff plan to keep nine offensive linemen on the 53-man roster, but if Johnson’s injury is serious and requires season-ending surgery, Lamar Holmes and Ryan Schraeder are the likely candidates to earn the job.
Johnson was supposed to replace Tyson Clabo, who was cut last spring to create salary cap space and signed with Miami.
The Falcons moved Johnson, a career guard, out to tackle to fill the void, but he did not have much experience. He started one of his 16 games last year — in a formation with three tight ends — and was limited to two games in his first two years because of injuries and difficulty in making the game-day roster.
Johnson was a third-round pick from Alabama in 2010.
A foot stress fracture limited Holmes, a third-round draft pick last year from Southern Mississippi, to one game as a rookie. Holmes believes Johnson’s situation won’t affect the offensive line’s unity.
“It’s all positive spirits,” Holmes said. “We really can’t speak on anything right now because we really don’t know yet.”
Schraeder joined the Falcons in April as an undrafted rookie free agent from Valdosta State.
“When you’re out there in the heat of fire, you don’t really think about that,” Schraeder said. “I just think about my job and what I’ve got to do to make the run the offense smoothly. Every day is a learning process, and hopefully I can keep moving in the right direction.”