The Glasgow youngster is one of two alterations made to the Dark Blues' line-up for this week's RBS 6 Nations encounter with Italy.
The 22-year-old replaces team-mate Alasdair Strokosch, who will visit specialists today to have the extent of an eye-socket injury assessed, while Ross Ford comes in for Dougie Hall after he limped off during Saturday's 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 inclusion elementary.
Also praising Ford, the Australian coach said: "Rob's form has been superb. He was bordering on selection initially. His form at Glasgow was great.
"He's got a great work ethic and the right to be angry being a red head. The fact is we need some contact dominance and he provides that. I'm really, really happy to have him back.
"And Fordy 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. It's an opportunity for him.
"Both the changes are enforced really. It was a short turn around with the prep that we had, so we said 'okay, we go again'.
"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 with a 23-18 win on Sunday afternoon, Scotland failed to live up to the demands of Johnson as they allowed England's pack to dominate at the breakdown.
The coach pulled few of his punches in the moments after the 38-18 defeat in London but he insists he was only harsh with his criticisms because he believes his men are capable of more.
He said: "I don't think it was strongly-worded, I think it was just honest. I don't want us to delude ourselves. We are where we are.
"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 and my thoughts after the game against England haven't changed to now.
"We know what the issue is and we are deluding ourselves if we say it is anything else. We just need to get better at it and I'm sure we will."
The frustration with Scotland's inability to compete with a physically superior Auld Enemy XV at mauls and rucks were only increased by the realisation that the team could hold its own when pacy backs such as Stuart Hogg, Sean Maitland and Tim Visser were handed early possession and room to roam.
The tries scored by full-back Hogg and New Zealand-born wing Maitland illustrated just that, but Johnson believes with a bit of work, his team are capable of surprising their doubters.
He said: "There is something in this team, as people and certainly as rugby players, and we have got the ability to put sides away. We have that strike power to do it.
"But we can't do that until we get certain areas of our game right. We will be focusing on the problem but if we get that right, watch out. On any given day, we can put any side away."
Johnson, who will give Hall time to prove his fitness ahead of Saturday's game at Murrayfield or else replace him on the substitutes bench with Glasgow team-mate Pat MacArthur, is also under no illusions about the quality of opposition Scotland will face next time out.
"I'm sure they will come and perform just as well as they did against France," he said. "They will be buoyant. It was a great success story. It was wonderful.
"The world order in rugby is changing. Professional rugby is right around the world now.
"The minnow countries of yesterday no longer exist, so every Test match you get is a competitive Test match. This will be no different.
"But guess what? That's what Test match rugby should be about. It should be a quality test of human endeavour.
"Italy are a quality side, they took Australia and New Zealand right to the wire in the autumn. So their result last week doesn't do anything to the respect we have for them.
"The question is whether we will do our part right."
Meanwhile, Harley has pointed to team-mates Kelly Brown and Johnnie Beattie as shining examples of the type of player he hopes to become.
The 22-year-old Glasgow back-row will make his first start for his country in the Dark Blues` next RBS 6 Nations fixture on Saturday as they host Italy at Murrayfield.
Harley made his debut in last summer`s tour of the South Pacific, adding a debut try to his one and only cap in the victory over Samoa, and is rated highly by interim head coach Scott Johnson.
He will join up with former Warriors colleagues Brown and Beattie at the rear of the Scots` pack after praising the pair - now playing with Saracens and Montpellier respectively - for their advice and support since he joined up with the national squad.
Harley said: "It`s been great coming into the team with Johnnie and Kelly already there.
"They have calmed me down a bit and handed me a few technical pointers. They have been keeping me right all week.
"Now I`m really looking forward to going into the game with them. I`ve played with Johnnie for a couple of years at Glasgow and I know the damage he can do to teams.
"Every session I`m picking this up from him and the others. I see the way he carries the ball and accelerates into the contact and that`s something I can pick up.
"I`ve never had the chance to play with Kelly when he was at Glasgow, so I`m looking forward to that. He`s a great example and I`ve looked to learn things from him too."
Harley has won honours with Scotland A and the Under-20 side but admits he will have to battle nerves in the build-up to his first start for full international side.
"Will I have any nerves?" he asked: "Yeah, definitely. Huge nerves.
"But I will try and use that to my advantage. I`ll be really nervous in the build-up but hopefully as I get towards kick-off I will be able to focus on what I`ve got to do and by the time I get out on the pitch, there won`t be so many nerves. That`s the plan anyway."
And the player refused to look ahead to nailing down a permanent place in Johnson`s team, saying: "At the minute I`m just looking at Italy and how they play. It`s about them and what they will bring.
"I`m taking it one game at a time. I don`t think you can look any further forward. It`s my first start so I will be fully committed to this.
"Italy are coming here off the back of a huge win against France and they will be full of confidence.
"But we have to match the intensity that they bring, because they will be really up for it."