That might be the case with Serbia, but you can't ignore his City form. He rarely looks intelligent on the ball, in fact his decision making has had me laughing several times, with his ridiculous cross and shot attempts from all angles. He will regularly get the ball in and around the box, sometimes near players and other times nowhere near, if anything his unpredictability of delivery is something of a weapon. Then there is the issue of his tendency to stand and watch instead of playing offside.
I don't think a player becomes massively good or bad depending on the team they play for, it's just that his City form has highlighted his glaring weaknesses. He seems to suit playing in a side that gives him a free run down the left side to play his own game, so in that sense he could well suit many matches Juve have played this season. Expecting him to be a well rounded player is something I don't.