"I have had to come back to Christchurch to work the babies because they are at an important stage and we need to look forward with them," he tells the Herald.
"So while I looked after the horses up there this week, I won't be going to Cambridge. That is a little bit of a shame because it is my home track and I'd love to have been there but this is my job now, I have to look after the whole team and delegate staff to be where I can't."
Cullen says it was a surreal feeling to see his name in the form guides next to Self Assured and Spankem.
"This isn't usually how it usually works. When you start training it is often with a few maidens and unraced horses, not New Zealand Cup and Miracle Mile winners. I don't think it has sunk in yet and I don't think it will for a few weeks."
So what does all this mean for punters? It would be unrealistic to think Cullen, or anybody else, could maintain the standards Purdon and Rasmussen have set so some horses might eventually see a dip in performance.
But there is no proof of that yet and if things change that should take a while to show up.
Cullen is a young man taking on a massive job and while he has been trained by the best as their foreman, he is still going to learn as he goes.
But the reality is Spankem and Ashley Locaz raced in the Auckland Cup only eight days ago so would have taken little work this week leading into a mile race, while Cullen says he is happy with Self Assured, who hasn't raced since winning the New Zealand Cup because of a corn.
"I worked him on Wednesday and he worked well," he says.
"He is not carrying any extra weight and is ready to go but the draws make it tough for all of ours because it looks like Copy That will lead and that will make him very hard to beat."
The Flying Mile at Cambridge being what it is, with leaders so hard to catch at this level, Cullen's debut with three of New Zealand's best pacers could well end in defeat because the best version of Copy That might be unbeatable in front.
But Cullen should get his first solo training victory — he has 62 in partnership with Brent Mangos between 2011 and 2014 — with Italian Lad (R8, No 11).
"He has worked well and is fitter for his last start so he will be hard to beat."
If he is right Cullen and his wife Amanda, who will help run the stable, can raise a glass for their first win in a unique chapter of New Zealand racing.
And then tomorrow get straight back to work.
FIVE HORSES TO BACK AT CAMBRIDGE
1. Son Of Patrick (R5, No 3): High drop in grade and has experience in mobile racing taking on the best 3yos at Addington last year. If he trots all the way he should win, as he did in his only previous start here.
2. Italian Lad (R8, No 11): Sat parked outside top 3yo B D Joe here last start and ran him close. Only here to get a penalty-free win before heading to Sydney.
3. Majestic Man (R7, No 6): Last time he raced in a sprint here he beat Sundees Son. Too short to back but Pick6 and multi anchor.
4. Hot And Treacherous (R11, No 7): Has been racing primarily in the elite 3yo pacing grade and falls into a perfect race. Almost another Pick6 anchor.
5. Santa Maria (R6, No 3): Latest outings at Alex Park have been in stronger maidens, has the gate speed to use her draw and the boss (trainer, Tony Herlihy) is on board.
