The duo were both injured in the 67-5 win over the USA, with Horne fracturing a cheekbone and McCabe suffering a dislocated shoulder.
To make matters worse for the Wallabies Anthony Faingaa (head), Wycliff Palu, Kurtley Beale (both hamstring) and Adam Ashley-Cooper (ankle) also picked up knocks in the 11-try rout of the Eagles.
Head coach Robbie Deans said: "Rob Horne picked up a cheekbone fracture but it's only a minor displacement, to use a medical term.
"That's promising. We will do some further investigation when we get to Christchurch and make a decision (on whether to seek a replacement) when we know more.
"Patty McCabe subluxed (partially dislocated) his shoulder. There again, we'll look at him over the next couple of days.
"We'll do some further investigation in Christchurch just to check on the full extent, but that in the first instance doesn't appear to be as grave as it potentially could have been."
Faingaa was carried off after a collision with Chris Wyles in the final minute of the match but Deans is pleased with the way he has responded.
Deans added: "He can recall everything, including the moments leading up to the tackle, so that's a great sign.
"But it's not something that we mess around with so we'll monitor him and just make sure he's fully recovered before he re-enters the fray."
Australia already have injury concerns over Digby Ioane (thumb), David Pocock (back), James Horwill (shoulder) and Scott Higginbotham (back).
Deans insisted Australia would not be rushed into making a decision on replacements.
He added: "We won't be making any decisions in the next probably 48 hours.
"We'll make sure that we're fully aware and fully informed before we move."
Experienced Wallaby Matt Giteau, though, is in the reckoning.
Asked specifically about Giteau, Deans added: "He's obviously in the frame for a potential call-up, absolutely. He was in the 40."
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