The Glasgow fly-half, who came off the bench during Scotland's 12-8 victory over Ireland on February 24, has impressed Johnson in recent weeks.
"Selection is tough just now but that's a good thing as we increase competition for places," ESPN quotes Johnson as saying. "Duncan did well when he came on against Ireland and he got some decent game time for Glasgow at the weekend.
"He's a talent with a really good work ethic and a thirst to improve, which I like a lot.
"Ruaridh remains a key part of our squad but he knows with Tom Heathcote proving a match-winner for Bath and kicking goals from here, there and everywhere, that he needs to up his game too."
Scotland are currently third in the Six Nations table, with four points from three games.