Murray, five times a runner-up in Melbourne, has struggled in the aftermath of hip surgery a year ago and is currently ranked 230 in the ATP standings.
The 31-year-old three-time Grand Slam champion returned to action last June but has won only eight matches since and recently admitted he still has pain from his hip and was not sure how much longer his career will last.
"In Australia, I would imagine he is just trying to get through without being in a lot of pain, so that would be sort of a victory," McEnroe told the Daily Mail.
"When you are playing best-of-five-sets, it is quite a bit different to best-of-three and that is a tall order.
"So, if he is feeling pain and not feeling right, it is hard to imagine he can go too long."
American great McEnroe, however, believes Murray will continue to strive to get back at the top of the game.
"Andy has missed the better part of a year and a half," he said. "I assume it is extremely frustrating for him, but I would be surprised if he would not give it at least another year or two to make sure he exhausts every opportunity.
"Who wants to walk away not on their own terms? Nobody."
Murray is playing in Australia under his protected ranking of world number two, which is used to help players who have suffered long-term injuries, but without being seeded faces a tough draw.
(Reporting by Martyn Herman; Editing by Christian Radnedge)
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