Armstrong initially struggled to make an impact in the Austrian's Saints side but has now become a key player on the pitch.
“Our history together was not the easiest one," Saints' manager reflected.
“He is a smart guy. He has a very clear opinion on where he wants to play. I had to speak lots of time with him to convince him about how the number 10 is the perfect position for him in my shape.
“A lot of players have their own view on where they should play, and it is important for every manager to convince the players about their strengths and what they have to learn.
“At number eight which he likes to play the most is, in our shape, like a 10 because the eights are playing a little bit more inside and they are not really wingers.
“This is the reason why he then accepts that this is a good position for him and he has, especially in the final third, some really good moves and can score always.
“From the physical part, he must really take care of his body because he has had some injuries also this season.
“He's a guy who is really investing a lot in the pitch and sometimes gets some light injuries that affect him for two, three weeks and it's always a setback and that's what we must avoid in the future."
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