Former Motherwell kid Johnston has been in previous Scots squads but is yet to win his first cap.
And Parensen admitted the 21-year-old is always doing extra work and that has been instrumental in him making his mark with the Austrian champions.
He said: "Max has good physical attributes. He's very fit, has a good engine and gets up and down the park. His pace, runs forward and delivery into the box is very important for the type of football we want to play at Sturm Graz.
"Max has become a very important player for us. He has a real professionalism.
"Max wants to work hard and to make himself the best he can be. After the training, he does his own indiv...