Johnston believes the player's versatility will be an asset.
He said: "It's good to get him. He's a quality player and he will be a good addition to the squad.
"For me, the big attraction is that he can play in numerous positions. He can play centre-half or central midfield.
"I'm sure he will do really well. We've got him for six months so it's a great opportunity for him to come in and play first-team football and I'm sure he will handle it no problem."
The 20-year-old Australian featured for Celtic just last week in a friendly against Liverpool in Dublin and put in a convincing display, encouraging the Killie boss that his new man is in form.
He added: "Seemingly he had a really good performance there. Hopefully he can take that kind of performance into our games."
Johnston has also recruited right-back Sean Clohessy ahead of Saturday's game against St Mirren and the manager is keen to boost his squad further.
He added: "We still need a few more and we are working on trying to bring a few more in.
"I don't know if it will be done for the weekend. Hopefully by next week we will bring two or three in."
The new Kilmarnock boss has begun his tenure with a defeat and a goalless draw but is confident of notching up wins and goals.
Johnson said: "The win will come, as long as we get performances like we got last week and we create the chances.
"You would be worried if we didn't create chances but we did create chances and with the likes of Kris Boyd and Paul Heffernan up front we will score goals."