The former Blackburn Rovers striker still needs to receive a final piece of paperwork from England before he is free to take up his playing duties with his new club.
United coach Miron Bleiberg said it was a frustrating situation as the Dutchman is fit and ready to go but cannot yet play against Central Coast Mariners this weekend because of red tape.
"Unfortunately on the eve of the game we are still waiting for the final piece of paperwork that will satisfy all the visa requirements, but although time is ticking there is still a chance that it will come through and he can play. It's a wait and see situation and that's all we can do," he said.
"We want Mace to play of course, but last week James Brown came in for him and we all saw what happened there. It was almost a given that James would score the goal because that's how football often works."