Tony McCoy is unlikely to make a decision on his ride in the Crabbie's Grand National until the final declarations are made on Thursday.
The record-breaking jockey is poised to choose between Irish challengers Colbert Station and Double Seven, both of whom are owned by JP McManus.
But McManus' racing manager, Frank Berry, told the Guardian that McCoy was expected to make up his mind "about half an hour before the jockeys have to be declared".
Berry said: "Double Seven wouldn't want a lot of rain.
"Colbert Station, any rain wouldn't matter to him, he'd handle any ground, which Double Seven wouldn't."
McManus is also likely to be represented in the great race at Aintree on Saturday by the Jonjo O'Neill-trained Lost Glory, but he seems unlikely to be on McCoy's radar.
Berry said: "He hasn't set the world on fire. It's hard to fancy him."
Kim Bailey, who won the Grand National with Mr Frisk 24 years ago, believes The Rainbow Hunter offers punters "unbelievably good value".
The 10-year-old gelding unseated his jockey at the Canal Turn last season but returns to Merseyside in fine spirits, having claimed the Sky Bet Chase at Doncaster on his last outing in January.
Bailey told the Telegraph: "He's not a Mr Frisk. For a start he's only 16 hands.
"He is very small, but he is agile and never made a semblance of a mistake last year. It wasn't his fault the jockey got knocked off.
"He has as much chance as anything else.
"He's in good form. I'm very happy with him and Aidan (Coleman) rode him in Lambourn on Friday and was happy with him.
"On the day with Mr Frisk we really thought he'd win. Everything was in his favour and when I saw him in the stable his eyes were bright and I could see he was in the zone.
"I'd never be as confident with The Rainbow Hunter, but we're going there without any great pressure.
"He was 50-1 last year, he's 33-1 this year and I think that's unbelievably good value.
"He's got class and he'll stay."
The forecast rain this week will give trainer Peter Maher optimism Big Shu can give Ireland a fighting chance of winning a first Grand National since 2007.
Maher's nine-year-old finished third in the defence of his cross-country crown at Cheltenham last month, but the County Kildare handler is anticipating further improvement at Aintree.
He told the Irish Times: "He came out grand. Good ground is not his business and it was a better race this year.
"He's always best on his third run. I think he was short at Cheltenham. He'd been sick a month before.
"Cheltenham was a remarkable run for everything to go against him. The ground went against him, things didn't go right, and Balthazar King is a good horse around Cheltenham.
"I was just so pleased to get him back in one piece after what happened during the week to the horses and jockeys that got hurt."