Osborne won the Ladbroke Gold Cup at Newbury aboard Lo Stregone and would have ridden the gelding last time out had he not been stood down for 21 days with concussion.
"Jamie is a super sub for Charlie Swan. He's ridden the horse a couple of times before and would have been in the saddle at Haydock," said trainer Tom Tate yesterday.
"Lo Stregone is in fine fettle at the moment. He is not a horse that needs a lot of work and is better fresh. He likes good ground but I don't expect Aintree to get very heavy."
Osborne is unlikely to select his Champion Hurdle mount, Mysilv or Collier Bay, until Monday morning, when the declarations for Tuesday's big race are made. His agent Gavin Davies said: "He is under no pressure from Jim Old (the trainer of Collier Bay) or Charles Egerton (trains Mysilv) and will leave it until he knows what the weather will do."