The former world number 17's ranking has crashed to 146 in an extended absence since Wimbledon, where he said he was "bored" with tennis after a first round defeat to Mischa Zverev.
Having pulled out of his last four tournaments, the Miami-based Tomic will face 19th seed Gilles Muller in the opening round at Flushing Meadows on Monday, a huge challenge for a player lacking match fitness.
A loss for the 24-year-old Australian, once touted a future top 10 player, will mean a tough road back to the big stages, said Masur.
"(It) leaves him where it leaves everyone else ranked 150 -- just having to get down into the Challengers and just fight, play hard," he said in comments published by Australian Associated Press.
"He’ll have no choice ... When you’re 150, you’re looking for (rankings) points. You can’t coast at any part of the year.
"It’s not quite as nice as going to Miami and Indian Wells, Monte Carlo and playing the 1000-level tournaments.
"And you’re playing this new breed of players," added Masur, "... players who are just incredibly hungry and they don’t respect reputation -- and you’ve got to earn it."
Six years ago, an 18-year-old Tomic stormed into the 2011 Wimbledon quarter-finals, becoming the youngest player since Boris Becker to reach the last eight at the Championships.
Pundits tipped grand slam titles would come with maturity, but he has not surpassed the fourth round at any since.
Tomic's off-court transgressions have drawn more headlines than his tennis, including a recent interview with Australian television in which he said he was only playing for money and had won plenty of it without really trying.
He bowed out of the U.S. Open's first round last year after a foul-mouthed rant at a heckler in the crowd, and was given the unflattering nickname of "Tomic the Tank Engine" after his second round capitulation against Andy Roddick in the 2012 tournament.
Masur said Tomic had the talent to be a top-five player but was not sure if he had the will to become one.
"You never know what’s in someone’s head. You really don’t. You can’t step into their shoes,” added Masur, Australia's former Davis Cup captain and a U.S. Open semi-finalist in 1993.
"He is freakishly good in aspects of the game. We know that.
"So can he do it? Yes he can. Does he want to do it? That’s the question. How badly does he want it?"
Reporting by Ian Ransom; Editing by Peter Rutherford