"I don't think he is any fitter than 85-90 per cent, at best," warns Purdon.
"He has had two trials and been really good but he is carrying a lot more weight all over his body than last season.
"Because of that I will drive him conservative and while that doesn't mean he can't win, I won't be taking off mid-race looking for the lead.
"So I'll be happy if he runs top three because he is still on the way up."
Purdon is naturally conservative and has erred on the side of caution before, only to be wrong and have horses win when he thought they couldn't, which hardly wins you fans among punters.
But the reality is he is always open about how he thinks his horses are going to perform.
So his words are best heeded, especially with Self Assured the favourite and the new standing start protocols in Canterbury, in which the horses stand rather than walk up, may not suit Self Assured.
If he drifts to $3, which is unlikely, Self Assured may be worth the gamble of everything going right but with key rivals such as Classie Brigade and Henry Hubert fit enough to make it tough for him, punters may be wise to steer clear.
Purdon then doubles down on the Self Assured doubt when comparing him with South Coast Arden, the northern pacer stabled at his property for the last two weeks.
"We haven't worked the two of them together but there is no doubt he [South Coast Arden] is the more forward of the two," explains Purdon.
"He is working well and Natalie has been doing a lot of the work with him and keeping in touch with Mango [trainer Brent Mangos].
"He could be a New Zealand Cup winner, he is that good, and even though he is on the unruly this week he can win."
Even if Self Assured can't give the boss a winning return tonight, Purdon suggests he should get a comeback win with Franco Mac (R11, No 11).
"He is a good juvenile, one of the top two or three for the Sires' Stakes so I think he will be very hard to beat."
Trots tips
Best bet: Sundees Son (R6, No 7): Huge fitness edge over Majestic Man and the small field should see him a comfortable multi maker.
Each way: Henry Hubert (R8, No 3): Not as good as the glamour boys but is fit and might sit handy on markers. His $8 opening quote was generous.
Ready to go: Franco Mac (R11, No 11): Trainer says he has improved on last season and second line draw no major concern.