TONY Cruz will decide on which horse he partners in the Derby after riding Northern Fire Ball in a dramatic last-minute piece of work this morning. Cruz was considered certain to ride Classic Trial winner Happy Guy for Brian Kan Ping-chee in Sunday's $3.5 million Blue Riband event with crack Australian jockey Darren Beadman taking the ride on last Saturday's impressive winner, Northern Fire Ball. But now trainer Wong Tang-ping has called on his old friend Cruz to partner Northern Fire Ball at trackwork. Five seasons ago Cruz made a last-gasp switch to partner Clear City to victory in the Derby, having previously committed himself to Illico. And last night Cruz said: ''At the moment I'm committed to Happy Guy and have told his connections so. ''But if I feel that Northern Fire Ball is a really good chance - better than Happy Guy - then I will partner him in the Derby. ''I've never been on Northern Fire Ball so I will be sitting on him tomorrow in a light piece of work and seeing what kind of feel he gives me.'' In other Derby news yesterday trainer Alex Wong Yu-on said the chances were bright that troubled favourite Deerfield would make the race. ''He is much better than 50-50 to run,'' Wong said about his American-bred hope who is suffering from a cracked hoof and will run in an aluminium hoof specially imported from Australia. ''The stewards have approved the aluminium plate. It should arrive on Wednesday and then it is up to the vets to see how he is after it has been fitted.'' Meanwhile Patrick Biancone's chances of securing the emergency retainer on top French rider Eric Legrix increased enormously last night with the news that champion New Zealand rider Lance O'Sullivan will not be able to fulfil his contract as a three-month Club Jockey. O'Sullivan has had to cut short his bid for a record eighth successive New Zealand championship to have a career-saving bone graft operation on his injured left hand. Speaking from New Zealand, a disappointed O'Sullivan said: ''This is the biggest letdown of my career. I was so looking forward to my three months in Hongkong and only hope the Jockey Club accept my reasons for not coming as totally genuine. ''I expect a full medical report to be faxed through to me by tomorrow. I will pass it straight to the Royal Hongkong Jockey Club.'' O'Sullivan injured his wrist in a fall at the provincial track of Te Aroha in exactly the same spot in the same race as he suffered a fall 12 months earlier.