Johnson, who went undrafted out of Texas A&M last season, played the entire second half in the Steelers' 24-23 loss to the Philadelphia Eagles on Aug. 9.
Tomlin indicated that is the plan again when Pittsburgh hosts the Indianapolis Colts Sunday.
"Even though he's been in an NFL training camp, I think he's only taken about four preseason snaps," Tomlin said. "We want to see what he is capable of, we want him get acclimated to an environment and give him a fair opportunity to see what he's capable of."
QB Ben Roethlisberger will start and Tomlin said the first-teamers will play "about twice as long" as they did against the Eagles.
Johnson was 3 for 6 for 93 yards with no interceptions and touchdown pass to rookie Chris Rainey.
"He's a young guy that's showing the growth and development that young guys show," Tomlin said of Johnson. "He's had some good days and he's had some days where he's taken a step back or two."
Anyone's possible: Tomlin said he's not ruling any player out for the Colts game other than the four players who are on the PUP list or just were taken off it Friday. RB Isaac Redman said he won't play Sunday. RB Jonathan Dwyer said he will.