Josh Hayes will be thrown in the MotoGP deep end after being called up to Colin Edwards in this weekend's Valencia season finale.
Edwards was injured during Marco Simoncelli's fatal accident at Sepang.
Hayes had been due to test the Monster Yamaha Tech 3 bike at Valencia next week, as a reward for winning his second AMA Superbike title for Yamaha, but will now make his MotoGP debut in the event itself.
The 36-year-old has no prior knowledge of the bike, track or tyres.
“I would like to say a huge thanks to Yamaha US, GYTR, the Monster Energy Graves Yamaha Team, the Monster Yamaha Tech 3 Team and Lin Jarvis from Yamaha Motor Racing for giving me this opportunity to race in Valencia," said Hayes.
"The circumstances leading to me being given this chance to ride the Yamaha YZR-M1 MotoGP machine are not how I would have chosen at all. My aim will be to represent Yamaha and Monster Energy in the best way I possibly can while trying to do Marco and Colin proud.
"Obviously I am facing a huge challenge having to learn a new circuit while also trying to the adapt to the bike and the Bridgestone tyres as quickly as I possibly can. I honestly don't have any expectations going into the weekend other than to enjoy myself and do a good job for Yamaha and Tech 3.
"I am certainly very excited to experience Yamaha's MotoGP bike for the first time and be part of the final race in the 800cc era."
Edwards underwent successful surgery on Tuesday at the University General Hospital in Houston to re-attach cartilage to his left humerus bone, which connects the shoulder to the elbow.
The four-week recovery period has prevented Edwards from participating in what was to be his last appearance for Tech 3 before he moves on to a fresh challenge - riding a Suter-BMW CRT bike for Forward Racing - in 2012.