The New Zealander said last week he would play a shadow Test side for the match at the Sydney Football Stadium but backline injuries and a few sore bodies for players who have played two matches in a week forced him to change his mind.
"We're still looking at combinations, we've told players there's an opportunity to stake claims on Saturday but the test team won't be selected until after next Tuesday," he told a news conference on Thursday.
"We haven't even spoken about the first test in terms of that. We've got a few knocks in the backs and the whole priority is that we arrive in Brisbane ready to go."
Winger Tommy Bowe was never going to play as he recuperates from a broken bone in his hand but England centre Manu Tuilagi has a shoulder problem, and fly-half Owen Farrell and winger George North are both carrying knocks.
Farrell will start on the bench with Jonny Sexton given the chance to cement his claim for the starting fly-half berth.
Ireland winger Simon Zebo, who was brought in as cover for Bowe, will get his first opportunity on the tour as will his compatriot Rob Kearney, who has been struggling with a hamstring problem and gets a place among the replacements.
Irish centre Brian O'Driscoll, who captained the Lions to a 64-0 victory over a Combined Country team on Tuesday, is also rested after doing the splits near the touchline late in the game in Newcastle.
"The thing with Brian is that we've got to manage him," Gatland said of the 34 year old.
"We spoke with him specifically at the start and said that we didn't really want to play him more than one game a week, and if at any stage, if his body was a little bit sore, we'd make sure that we kept him fresh and didn't tire him out."
In his absence, the Welsh combination of Jamie Roberts and Jonathan Davies start in midfield, in front of compatriot Leigh Halfpenny at full-back.
Tour captain Sam Warburton will lead the side out for the second time after making his Lions debut in the 22-12 victory over the Queensland Reds last weekend.
After playing in an all-Welsh back row against the Reds, Warburton gets the chance to gel with Irish number eight Jamie Heaslip and English blindside Tom Croft.
"It's always nice to have game time, I always feel that the more games I play, the better I play," Warburton said.
"You get a feel for the pace of the game around the contact area and yeah, I'm looking forward to it.
"Really looking forward to playing with Tom and Jamie, you want to come on Lions tours and play in different combinations."
The injury problems in the backline were the reason why England centre Billy Twelvetrees had been called up from Argentina overnight, Gatland said.
"We know he can play 10 and he can goal kick as well but we felt that we had a lack of midfield cover," said Gatland. "And when we looked through the side, there was potential for some of those players to be going into a fifth game in a row.
"And we need to keep some of those players fresh for the Test."
After Saturday, the Lions have one more tour game against the ACT Brumbies in Canberra on Tuesday before the first Test at Lang Park, Brisbane on June 22.
Team: 15-Leigh Halfpenny (Wales), 14-Sean Maitland (Scotland), 13-Jonathan Davies (Wales), 12-Jamie Roberts (Wales), 11-Simon Zebo (Ireland), 10-Jonathan Sexton (Ireland), 9-Mike Phillips (Wales), 8-Jamie Heaslip (Ireland), 7-Sam Warburton (Wales, captain), 6-Tom Croft (England), 5-Paul O'Connell (Ireland), 4-Alun Wyn Jones (Wales), 3-Adam Jones (Wales), 2-Tom Youngs (England), 1-Mako Vunipola (England).
Replacements: 16-Richard Hibbard (Wales), 17-Alex Corbisiero (England), 18-Dan Cole (England), 19-Geoff Parling (England), 20-Dan Lydiate (Wales), 21-Ben Youngs (England), 22-Owen Farrell (England), 23-Rob Kearney (Ireland).