Newly-crowned MotoGP world champion Nicky Hayden will miss the final test of the year to return to the United States for possible surgery to his right shoulder.

The MotoGP Web site (www.motogp.com) said the Kentucky rider had aggravated a shoulder injury at the penultimate round of the season in Portugal in a collision with Honda team mate Dani Pedrosa.

Hayden, 25, had kept quiet about it so as not to give Italian title rival Valentino Rossi, who went into the last race in Valencia with an eight-point lead, any more of a psychological advantage.

Rossi crashed in the Spanish race, ending years of domination of the top category and handing Hayden his first championship.

"No date has yet been set for possible surgery, which may entail the removal of a metal plate previously inserted in his shoulder," the Web site said.

The final test of the year is in Jerez, in southern Spain, next week.