Johnson is the leading candidate to step in for veteran Madieu Williams, who could be lost for up to six weeks because of a nerve problem in his neck.
The loss is a definite setback for a team that made Williams one of its top off-season priorities. The former Cincinnati Bengal was signed to a six-year, $33 million deal that included nearly $13 million in guarantees early in free agency.
A favorite of Vikings defensive coordinator Leslie Frazier, who served in the same role with the Bengals during Williams' rookie season in 2004, Williams has played cornerback at times during his career and is considered a top-notch cover guy. That was important for a Vikings pass defense that finished at the bottom of the NFL the past two seasons.
Veteran Dwight Smith had been starting opposite Darren Sharper but was released last winter as the Vikings looked to upgrade.
One benefit of having Williams was that his ability in coverage potentially could enable Sharper to have some freedom to freelance. Sharper tied his career-high with nine interceptions in his first season with Minnesota in 2005, but has eight picks in two seasons since Brad Childress took over as coach and brought in coordinators (Mike Tomlin and Frazier) who use the Tampa-2 as their base scheme.
It remains unclear exactly when Williams injured his neck but it became obvious something was wrong when he sat out the preseason opener against Seattle with what Childress called "neck spasms."
Childress said he doesn't expect the Vikings will pursue a safety to replace Williams, meaning they almost certainly will continue to turn to Johnson, who has started the first two preseason games.
Second-year player Eric Frampton also has gotten some work with the first team, but he is viewed more as a special-teams guy.
Johnson, who started against the Seahawks and got snaps with both the first and second team, said he feels "well prepared" to move into a starting role. The Vikings certainly are hoping that's the case.
If Williams does end up missing six weeks, that means he would sit out the first three games of the regular season. The Vikings open at Lambeau Field on Monday night, Sept. 8, before returning home for games against Indianapolis and Carolina.
Sharper is confident Johnson can contribute and said the key thing for him will be to be in the proper position. Nonetheless, it will be a tough transition for the four-year starter from Arkansas State.
"With what we like to do, we have to make sure everyone is in a certain area and you have a certain area you have to cover," Sharper said. "You never have to do anyone else's job.
"That's probably the main thing. Being a star player in college, a lot of times you're asked to do a lot more things. Here in the NFL, with the defense that we have, you just need to do your job and everything will take care of itself."
QUOTE TO NOTE: "That ain't going to happen Monday night (in the opener). Trust me it ain't going to happen. I've got everybody focused." Vikings nose tackle Pat Williams, promising Packers running back Ryan Grant will not have another 100-yard rushing performance against the Vikings in the regular-season opener like he did last November at Lambeau Field. Grant was the only running back to gain 100 yards on the ground against the Vikings in 2007.