Evans will win his 42nd cap for Scotland in Cardiff with Jack Cuthbert providing back three cover on the bench.
And while Fife gets his chance as a replacement for Seymour, who has not recovered from an ankle injury sustained against France, Johnson believes he can stamp his mark on the occasion.
"We like Dougie's size and his competitive edge. He chases everything, which I like, and he has shown really good glimpses playing for Edinburgh this season," said Johnson.
"He's been ear-marked by us for a while - we've been following him for the last 12 months - and he's got his reward now. If he gets this one right, you might find that he puts a bit of pressure on (for a place) going forward, which is really good.
"I don't really think Saturday will faze Dougie. He's one of those players where it doesn't seem to matter who he marks he just goes out and plays the same."