"I would love to think he could win but it might be nearly impossible," said Purdon.
"If it was over 2700m I'd give him a good chance but over 2200m if they step and run hard off the front they could pace something like 2:45, which would leave us needing to pace 2:40 coming wide.
"That is a bit unrealistic so we are going to need a lot to go our way to be honest."
The best scenario for Pembrook Benny would be a few early gallopers to reduce traffic later, while he makes a lightning quick beginning and is able to latch on to the back of the bunch before the 1600m mark.
If that unfolds he could win but Purdon has been around long enough to know the wonderful 9-year-old pacer could produce an enormous performance and still get beaten.
"But he needs the racing because he has a $25,000 race at Addington next week and the Easter Cup after that.
"I am not saying he can't win because you like to think they always can. But it will be very hard."
Speedy mare Spirit And Desire, who can begin quickly and is off a 20m handicap, so 45m and valuable field position ahead of Pembrook Benny, looks the best winning chance.
While the handicap could be beyond Pembrook Benny, Purdon's confidence is understandably much higher with Sky Major (R4).
He has been brave in the red hot Derbys this season and drops a long way to take on a few speedsters here, but should be too good as he prepares for the NZ Derby in the absence of stablemate Maxim, who is out for the rest of the season with injury.
The night's other highlight is the clash between Bettor Be Supreme and Unforgiving in the $19,000 Sales Series Graduette as the juvenile filly season starts to hot up.
Bettor Be Supreme won her debut over a wayward Unforgiving last Friday night.
She looks a top-class act for in-form trainer Brent Mangos.
