The Rebels will resist making a call on their starting line-up until game day, hoping for injury doubts to ease over a few players.
They have taken hard-hitting Wallaby Lopeti Timani to Dunedin, in the hope the lock might play after being sidelined since a round-two knee injury.
They also have concerns over two other locks in Welshman Dom Day and Kiwi Culum Retallick.
"We still haven't decided on our 23, and we're giving everyone every opportunity to get right and that will extend all the way up to tomorrow."
A horror injury toll against the Waratahs contributed to the Rebels' second-half fadeout at AAMI Park, where they blew a 19-point halftime lead. McGahan hoped to be able to utilise the strength of his bench late in the Highlanders match.
"We've seen more and more the impact of benches, and what you need to get done to play out the full 80 minutes.
"You see that, results-wise. Some teams have won a couple right at the death and we've lost a couple."
While the Rebels sit last in the Australian conference, the Highlanders hold the same position on the Kiwi ladder, although they have at least banked two wins.
Melbourne feel well-versed in New Zealand tactics, after already taking on the Blues, Hurricanes and Chiefs this season.
Despite their position, McGahan said the Highlanders would be far from easy, proven with their last-round win over Aussie leaders the Brumbies.
"They won a title a couple of years ago and have been thereabouts since," McGahan said. "They've got a couple of world-class players and they work hard for each other, and they don't like losing down here."