Ulster jockey Peter Buchanan will ride Big Shu in the Aintree Grand National next Saturday for local owners Caroline Woods, Richard Robinson and Hugh Duffy.
Big Shu signalled Grand National potential by winning the Glenfarclas Handicap Chase at last year's Cheltenham Festival.
Tony McCoy – who had his only National victory in 2010 on board Don't Push It – is tipped to ride Double Seven following Sunnyhillboy's withdrawal yesterday.
"I am not sure what horse I will be riding," said McCoy last night.
Sunnyhillboy was runner-up by the smallest margin in the 166 runnings of the race two years ago when going down by a nose to Neptune Collonges.
Trainer Jonjo O'Neill took the decision to miss the big race after the 11-year-old failed to shine in a piece of work yesterday.
"He's been a super horse for us but he just didn't sparkle in his work," said O'Neill.
"I had a word with Frank (Berry, racing manager) and JP (McManus, owner) and we've called it a day (for this season).
"Going for a race like that you have to be spot on and he just hasn't done what we'd have wanted in his last few bits of work."
Sunnyhillboy had been a possible National ride for Ulsterman McCoy, and his choice now appears to rest between Colbert Station and Double Seven.