Amini was the third player cut by the A-League Men club in the space of 11 days ahead of the January transfer window, joining Ecuadorian defender Luis Canga and Colombian forward Cristian Caicedo.
While Canga and Caicedo left by mutual consent, the Glory said they would not be making any further comment about the nature of Amini's departure in a brief three-sentence statement.
The Professional Footballers Australia (PFA) confirmed on Monday Amini would initiate legal proceedings against the club, claiming his termination was unlawful.
"Whilst we are confident of a successful resolution for Mustafa, recent seasons have seen a rise in several clubs attempting to deploy abusive practices aimed at forcing players to waive their contractual rights," PFA boss Beau Busch said in a statement.
"Mustafa is hugely respected by his peers across the league, and we will ensure he is fully supported during this difficult time for him and his young family.
"In the absence of an effective A-Leagues governance model that ensures uniform club compliance with the Collective Bargaining Agreement, the PFA will consider alternative responses to ensure the players' rights are respected across the league."
Amini, a former Socceroos midfielder, has not featured this season for the bottom-two side due to a knee complaint.
The 31-year-old joined Perth in 2022 and played 19 games in his first season and 17 the next.
His most recent match was on April 28 last season, when Glory suffered a 7-1 hammering from Sydney FC.
Under new coach David David Zdrilic, the Glory have only claimed one win and two draws from 10 games this season.
After defeating fellow strugglers Brisbane in round nine, they were thumped 3-0 by Macarthur on Friday night.
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