He instantly became a permanent member of Fiorentina's starting line-up, and he came close to winning the Scudetto during his first season with the club, missing out on the title to Juventus by a single point.

He was also Milan's top scorer in the 1993–94 Serie A season with 11 league goals, helping them to win their third consecutive title since 1992 under Capello.

[10] He was a member of the Italian squad that won the 1982 FIFA World Cup in Spain, but did not receive any playing time during the tournament.

[9] Massaro played between 1984 and 1986, but was called up for Italy's 1994 FIFA World Cup squad by manager Arrigo Sacchi at the age of 33.

[11] This was his only goal for Italy,[9] making him the country's oldest-ever goalscorer at the FIFA World Cup at the age of 33 years and 36 days.

[12] In the 1994 FIFA World Cup final loss against Brazil, he missed a one-on-one opportunity and later failed to convert a penalty kick in the shoot-out, which marked his last international appearance.

[3] Massaro was only utilised as a true striker later on in his career, in particular under Milan's subsequent manager Fabio Capello, and also at the 1994 World Cup with Italy under Sacchi, where he was able to find the net more often, due to his finishing ability with either foot, as well as his aerial prowess and heading accuracy;[17] in this position, he was also capable of playing off his teammates and of holding up the ball when playing with his back to goal.

[1][2][16] Describing his career experience at Milan, Massaro once referred to himself as a "supersub," due to his penchant for scoring decisive goals after coming off the bench.