This was the horse that won the first two legs of the final in Helsinki last year, before dropping back to ninth overall with a total of 20 faults in the final two-round contest.
"It was a disappointment in the end, but not a major one," Whitaker said. "I was surprised to win the first two parts and we might have hung on if the courses for the last one had suited Grannusch - but they couldn't have been worse for him."
Both tracks included a triple bar (a spread fence of three rails of ascending height) within a combination. Grannusch has always had problems with this type of obstacle and so it proved again.
Since then Whitaker has decided that he will not subject his older horses to the same sort of pressure. He intends to switch to 10-year-old Heyman tomorrow evening and on Sunday afternoon, when the last two legs of the contest are staged.
Though still "a little bit unpredictable", Heyman's recent efforts in Dortmund (he was second in the Grand Prix there and 10th in the World Cup qualifier) suggest that Whitaker is building another effective partnership. The longer he has a horse, the greater the rapport he usually achieves - as witnessed with his sprightly veterans.
"You have to treat all the horses as individuals and try to find the right key, without forcing them," he said. "That has to include knowing how to feed them and what sort of work they need." Riders know most of that, of course; achieving it is quite another matter.
Whitaker is helped by having an unerring eye for a stride and an equable temperament. You never see him being ratty with a horse when it has made a mistake. "It doesn't do any good. You need to build up trust and confidence, which means you have to control your temper and try to analyse what went wrong."
Gothenburg is one of Whitaker's favourite venues. "The atmosphere is always good and I've been lucky there," he said. "Fortunately I seem to be quite popular with the public, which is nice, too."
This is the closest the 43-year-old Yorkshireman is ever likely to come to sounding boastful.
The Swedish city is the original home of the World Cup final - and also of Volvo, the firm that sponsored it for 20 years. Many of those who watched the inaugural final of 1979 have returned every year (there have since been eight other finals and 11 qualifiers in the Scandinavium), often requesting the seats that they originally occupied.
This knowledgeable audience has therefore had plenty of opportunity to appreciate Whitaker's quiet wizardry in the saddle. He is the only rider to have qualified for every final (last year's wild card having proved unnecessary when a couple of others dropped out) and he has missed only one - at Del Mar in California in 1992, when it was decided to save Milton for the Barcelona Olympics that summer.
He has won the final twice, been runner-up three times and finished in the top 10 on another seven occasions.
Whitaker has a soft spot for all the horses who helped him to achieve this remarkable record - from Ryan's Son ("he got me started") and the fabulous Milton ("who took me a stage further") to the present stars of his string. "You always get attached to the ones who try for you."
This year Whitaker and his younger brother, Michael, will be the only British participants because Geoff Billington, their regular team-mate and drinking companion, narrowly failed to qualify.
The Whitakers' opponents will include two of the greatest horsemen ever: Germany's Ludger Beerbaum (the European champion) and Mexico's 26-year- old Rodrigo Pessoa (the holder of both the World Cup and the World Championship title). We can expect to see homemade banners proclaiming love for these two - but there will also be some for John Whitaker, who still has a small army of loyal supporters in Sweden.