The 21-year-old defender/midfielder, signed recently from Hyundai A-League club Newcastle Jets, was one of the stars in the Italian club’s 1-0 win over the Argentine's in the capital.
Mario Gomez' goal on the half-hour was the deciding goal of the friendly, La Viola's first of three games in South America.
Argentinean journalist Juan Pablo Varsky was impressed with the former Young Socceroo, calling him a "gem" of a player.
This comes after Fiorentina coach Vincenzo Montella sensed the fringe Socceroo was ready for the game.
"Josh Brillante is ready physically and I consider him to be a nice surprise," he said earlier in the week..
Fiorentina will play Brazilian side Palmeiras before facing Peruvian side Universitario in their final games in South America.