The Ukrainian, 35, had joined Ukraine’s territorial defence forces to defend the nation against the Russian invasion.
However, he recently left his home country to start training for his second bout against the Brit, according to reports.
But he has received the backing of the Klitschko brothers to leave the war effort in his homeland as he prepares to defend his belts against Joshua, whom he dethroned in September.
And older brother Vitali has offered his fellow countryman some words of advice ahead of his biggest fight to date.
Vitali told The Telegraph: “A very important message could be carried through an event like this.
“We do not know if in three months we will exist.
“I actually have plans to speak to Oleksandr Usyk to give him advice if he decides to make the fight, to have some special messages that he can put out there.”
Wladimir, Vitali’s younger brother, famously used Usyk as a regular sparring partner during the latter stages of his career.
And he understands how difficult it must have been for Usyk to decide to leave his family behind to fight Joshua.
He added: “There are pros and cons and it’s a hard decision to make for Oleksandr Usyk but to have the Ukrainian flag raised and our anthem played and one of our ambassadors of our country out there in the world with the right mindset could be more positive than negative.
“We need support. Since the beginning of the war, the economy is down to nothing, but please don’t forget we are defending democratic principles of freedom and free choice.”
Joshua’s promoter Eddie Hearn has confirmed that he will bid to regain his old WBA, IBF, WBO and IBO heavyweight belts in June and revealed that they have also received offers from the Middle East and United States to host the fight abroad.
However, the rematch would take place in the United Kingdom if it were up to him, but unfortunately the decision is out of his hands.
Hearn told DAZN: “Obviously now everybody’s made their statements, Usyk’s come out and said, ‘I’m starting camp,’ so that fight is a go for June.
“We’re now going through all the various offers from different countries to stage that fight, with [Usyk’s promoter] Alex Krassyuk.
“The Middle East is obviously very interested in that fight, we’d like the fight in London as well if possible.
“I think probably this week, early next week, we’ll start firming up some dates. But, as I said: Usyk-AJ rematch definitely to take place in June.”