Batty had to have a small unexpected operation on his hip, which precludes the in-form keeper from Southern's defence in the penultimate ASB Premiership match of the season.
Coach Luiz Uehara was absent, farewelling injured Brazilian import Henrique Viana, who flew home yesterday, and Hart addressed what was left of the United squad in heavy rain at Tahuna Park, trying to lift spirits for the Dunedin match.
''We really need to start well, and apply pressure from the starting whistle, or this season could end in an anticlimax, after all the hard work we have put in,'' Hart said.
Certainly Auckland, which won the first-round match 3-0, will still have its foot firmly on the accelerator as coach Ramon Tribulietx urges his team on to clinch a place in the OFC Champions League, which it has done for the past decade.
In a classic ''haves and have-nots'' situation, Auckland is likely to field its full experienced squad, plus a new signing, Portuguese import Joao Moreira.
In contrast, Southern United surveys gaps in its starting line that ideally would have included Viana, former Kiev midfielder Patrick Ebanda, centreback Matt Joy and Batty.
''We are trying to get reserve keeper Jefferson Potrich to play, but he had already informed us of work commitments - which we now hope can be changed,'' Hart said.
''Otherwise it's going to be a huge step up for one of our youth keepers, such as Tom Stevens or Josh Dijkstra, against the power and experience of Auckland, which has half a dozen players over 30 years old in their squad and is full of internationals.''
Centreback Peter Overmire and fullback David Hayman have already been added to the United squad, and Aajay Cunningham and Henry Flood left out. That is unless further changes are made at today's light training and fitness check.
Southern has had its moments this season - the 3-1 win over Waitakere will long be remembered - but since then the team's defensive statistics have slumped. It has conceded more than three goals per match, 37 goals in 12 games.
Yet Hart is adamant United can compete with Auckland, and Wanderers in Invercargill next week, if the team regains its confidence and spirit.
''Our best form can upset any team in the league. We just have to switch it on.''
Today, the Southern United youth team plays Canterbury in a national league fixture at 12.30pm at Tahuna Park.
Like the senior side, the youth team has its problems with defections and injuries. Midfielder Matt Kelly is out for the season with knee ligament problems, Joe Clark is also injured, and Keegan Taig is suspended after two yellows last week.
Canterbury is second-top of the Southern Conference and, six points behind, Southern is bottom.
Southern v Auckland: Forsyth Barr Stadium, tomorrow, 1pm
Southern United: Jefferson Potrich, David Hayman, Peter Overmire, Campbell Attwood, Nicki Hindson, Morgan Day, Sam Mepham, Aajay Cunningham, Andrew Ridden, Regan Coldicott, Taylor McCormack, Geordie Mansford, Peter Rae, Daniel Morris, Campbell Higgins.
Auckland City: Tamati Williams, Takuya Iwata, Angel Berlanga, John Irving, James Pritchett, Chris Bale, Darren White, Daniel Koprivcic, Nicolas Zambrano, Adam Dickinson, Ivan Vicelich, Joao Moreira, Louie Caunter, Emiliano Tade, Rory Turner, Andrew Milne, Sam Burfoot, Oliver Sail.
