Spanish media reports said the move could cost Juventus Sh12.4 billion (105 million euros, $120 million), with Ronaldo signing a four-year contract worth Sh3.5 billion (30 million euros) per season.
The USB union said that the £99 million required to sign the 33-year-old player to Juventus should have been invested to safeguard the future of Fiat's future of thousands of people, "rather than enriching only one".
Tacchinardi began by hailing Ronaldo's arrival stating that: "He certainly gives a new image to our football".
Cristiano Ronaldo's decision to quit Real Madrid might have come as a big surprise to many, but not to the player himself. "The company should invest in vehicle models that guarantee the futures of thousands of people, rather than enriching only one".
Juventus would need to sell before they can add to their squad following suggestions that they are looking to offload one of their strikers to fund Ronaldo's move.
Sagan Wins Stage Two at Tour de France, Dons Yellow Jersey
The Sunweb man won the points race in Paris a year ago largely due to the fact that two key rivals didn't make it that far. It was climbing a little bit in a headwind and already the last five kilometres were up and down.
"The Champions League? Obviously Cristiano will bring us quality. That means more sponsors, more TV money and more prize money".
Modric, who is set to captain Croatia in the World Cup final on Sunday, conceded his disappointment over the departure of the Portugal great.
To drum home their concerns, the USB union has called for a strike at the Melfi plant in southern Italy. Because Juventus and Fiat Chrysler are partly controlled by the same owners, the announcement has triggered a union strike at Fiat Chrysler's Melfi plant in Italy.