Mitchell played little part in this year's club season but was initially named in the Stags' 2018 Mitre 10 Cup squad.
However, last year's captain has since received news he requires surgery on his neck, which has ended his hopes of lining up in this year's competition.
Mitchell will continue to help the Stags where he can in a non-playing role this season.
It prompted Stags coach Dave Hewett to look at various options to fill the void left and it was in Australia where he found his man in Ready.
``I've spoken to [Reds coach] Brad Thorn about him and talked to Andrew.
``He's really excited about it. I know him through my involvement with another team and he was a bit of a menace.''
Ready joined the Stags set-up yesterday in preparation for the Stags' opener against Hawke's Bay in Invercargill on Sunday.
With Mitchell ruled out the captaincy duties had to be filled as well.
The captaincy in pre-season was shared by young hooker Flynn Thomas and veteran back James Wilson, and that will remain the case during the Mitre 10 Cup.
It is a big assignment for Thomas, who just this year lined up for the New Zealand under-20 team.
Thomas talked with his parents, senior players and former first XV coach Peter Skelt before accepting the offer from Hewett.
``It was a big surprise. I was a bit hesitant, but after a bit of deliberation and a few chats, I came to the decision that it was the right thing to do,'' Thomas said.
``I felt like I hadn't earned the right to take on that role, but it's a real honour.''
Thomas will lead the forwards and Wilson the backline, Hewett said.
Wilson, who has returned to Southland after a six-year stint overseas, is pleased he could play a leadership role in the team.
``It's a huge honour, representing your home team.
``Being away and now coming back, I wanted to play a leadership role and offer the experience I've gained over the years, but doing so with the captain's badge is an opportunity that I'm very grateful for.
``This is a good bunch of lads who just want to learn, so that makes the job a lot easier.''
