The off-contract 23-year-old defender has been enjoying his most productive season with the Sky Blues since making his debut in 2013.
And the club were keen to lock him down on a multi-year extension in recognition of his evolution.
However, Sydney-raised Calver has been wooed by a switch to Melbourne to join the fledgling entity who, it is understood, have already locked down a dozen signings for their debut season.
Three have been officially announced - defenders Josh Risdon, Connor Chapman and Greek marquee striker Panagiotis Kone.
The Central Coast Mariners pair of Connor Pain and Johnathan Aspropotamitis have also agreed to terms.
One-club man Calver has made 15 appearances this season - a total of 49 in the A-League - and his impending departure is a blow to coach Steve Corica, especially since veteran stopper Alex Wilkinson is deep into the autumn of his career, and will need to step aside at some point.
Western United are expected to reveal further names in the coming weeks as their football department cranks up their recruitment.
