The USB union, which represents only a small number of workers, called a strike at Fiat Chrysler's Melfi plant in southern Italy after Italian champions Juventus agreed to pay Real Madrid 100 million euros to secure the Portuguese striker for four years.
Both Fiat Chrysler (FCA) and Juventus are controlled by Exor, the investment holding of Italy's Agnelli family, which owns nearly 30 percent of the carmaker and 64 percent of the soccer team. However, the two are completely separate entities, each managing their own finances and operations.
USB was protesting over the fact that tens of millions of euros were spent to hire Ronaldo while workers at FCA were being asked to make sacrifices.
The strike was called for late Sunday to early Tuesday.
The bigger unions criticised USB's move as "mere advertising," saying it was "incredible that in such a delicate phase for the FCA group... someone is calling for a strike on a matter that has nothing to do with work and with workers."
Thousands of FCA plant workers across Italy have been on state-sponsored temporary layoff schemes for years due to lack of new models.
However, the company has launched a new investment strategy to 2022 which it says should allow all employees in Italy to return to work over the next four years.
FCA, whose plants churn out Jeep SUVs, sporty Alfa Romeos, luxury Maseratis and tiny Fiats, had promised to bring all employees in Italy back to work by the end of 2018, but that goal slipped as investments and product launches were delayed.