"He worked so well on Sunday morning I thought he could win but he was lame on Sunday afternoon," said Purdon yesterday.
"We have worked on it and he was a lot better today than yesterday and if it keeps improving at that rate then he might start.
"But it will all come down to how he is in the morning.
"If he trots out well then he will start, if he doesn't we are not going to take any risks with him."
The dilemma mirrors that of last season when Purdon was forced to scratch Auckland Reactor from the Cup with almost exactly the same injury.
"It is stressful and I can't believe it's happened again."
Until the setback, Purdon rated Sushi Sushi the best of his record five representatives in the Cup.
"He had kept improving with every start he had for us and his manners are great," said Purdon.
"So if he starts and we see the best of him then he can still be a real chance.
"But these small setbacks don't help."
Of his other Cup starters, the fact Purdon jumped off long-time drive Auckland Reactor to partner Fly Like An Eagle shows where they rate, although both have standing-start concerns.
"If Fly Like An Eagle steps away safely he is a chance, but he is still learning and being a 4-year-old I'd like to drive him with a sit.
"Auckland Reactor looks well but, really, I don't know what to make of him. So we are hopeful without being confident there."
Highview Tommy shocked everybody with his enormous third in the Cup last season but his lead-up form has lacked sting, while Major Mark has never looked good enough to win a New Zealand Cup so victory would surprise.
What would surprise nobody, though, is Purdon's champion trotter I Can Doosit making it 16 wins on end by overcoming his outside draw in today's Trotting Free-For-All.
"He is very, very well and I will look to go forward on him and get handy," said Purdon.
In the absence of arch-rivals Stig and Dr Hook, only bad luck looks capable of beating the champ.