I think it was the system at Southampton, when Koeman was at Southampton they played some great football, when Puel, Pellegrino and Hughes were in charge they played a terrible defensive style which didn't suit Tadic at all. Now he is at Ajax he is really about to showcase his attacking ability again, plus he has lived in the Netherlands for a number of years before Southampton, he actually said he prefers life in the Netherlands than England in an interview as that was his reason for going back to The Netherlands.