Johnson, listed first on the depth chart at his position, was injured on a running play Tuesday. Johnson was carted off the practice field and onto an ambulance. He's scheduled to have surgery next week, Smith said.
A league source told ESPN NFL Insider Adam Schefter that Johnson is expected to miss the entire season with the injury.
Johnson 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.
The Falcons and Cincinnati Bengals were practicing together for the second straight day. It's the second consecutive significant injury by a Falcons player in the practices between the two teams. On Monday, starting linebacker Sean Weatherspoon suffered a a dislocated left finger that resulted in an open wound.
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.
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."
Information from The Associated Press was used in this report.