The Glasgow Warriors youngster is one of two alterations made to the Dark Blues' line-up for Saturday's RBS 6 Nations encounter with Italy.
The 22-year-old replaces team-mate Alasdair Strokosch, who will visit specialists today to have an eye-socket injury looked at, while Ross Ford comes in for Dougie Hall after he limped off during the Calcutta Cup clash against England at Twickenham with a knee knock.
But Johnson claims the impressive displays of Harley in the RaboDirect Pro12 this season made his decision simple.
He said: "Rob's form has been superb. He was bordering on selection initially. His form at Glasgow was great.
"The fact is we need some contact dominance and he provides that. I'm really, really happy to have him back."
He added: "Fordy is coming back in too as an experienced player.
"Dougie was a bit unfortunate, he got a bit of a bang in the England game. But that gives Ross another chance to come in, and it's an opportunity for him.
"Both the changes are enforced really. It was a short turnaround with the prep that we had.
"It's an opportunity for some to put down a marker. We are still in this tournament after the results of last week, it's up to us now."
While Italy summed up an impressive display of power and commitment to stun France 23-18 on Sunday afternoon, Scotland failed to live up to the demands of Johnson as they allowed England's pack to dominate.
He said: "We need to be good, just look at Italy's form, they were terrific. They did really well against a great team like France.
"But we are not deluding ourselves. We want better effort. The game, once again, will be based in the contact area.
"We know what the issue is and we are kidding ourselves if we say it is anything else."