The 30-year-old France international had criticised the move as "crazy" earlier this week, prompting De Siervo to respond on Wednesday and say that footballers should respect the money they make and their employers.
Serie A has turned the unavailability of the San Siro - home stadium of Milan - in February due to the Winter Olympics into an opportunity for Italian football to increase international visibility, with the fixture against Como to be played in Perth.
"Rabiot's observation is shared by the vast majority of players because it is accurate, because it is based on empirical knowledge of football and the profession of footballer," UNFP said in a statement on Thursday.
"Not all of them have access to the media to express themselves in this way.
"However, they should know that as the main players in the game, they have the right and even the duty to make their voices heard.
"Freedom of expression for footballers is a fundamental principle that the UNFP will always defend, as will FIFPRO, the global players' union."
European football's governing body, UEFA, is against domestic league matches being played abroad and reluctantly approved the fixture on Monday. UEFA has also approved a LaLiga match in Miami in the United States, saying FIFA's regulatory framework, still under review, lacks clarity and detail.