The Factory Yamaha Team has officially confirmed that Ben Spies will miss this weekend's Australian MotoGP at Phillip Island.
Spies fell heavily during Sunday's wet Malaysian Grand Prix at Sepang.
“An initial check by the trackside medical centre found no serious injuries however further tests conducted today in Kuala Lumpur have revealed injuries requiring surgery,” said a Yamaha statement.
“The Texan will return to the USA tomorrow, Tuesday 23rd October, to receive treatment for an AC shoulder separation, a cracked rib in the upper chest area and bruising to the lung.
“Yamaha Factory Racing will not field a replacement rider at Phillip Island due to time constraints.”
Spies first revealed that he would miss this weekend's event on Twitter
, marking the second time he has bypassed Yamaha after also announcing he would leave the team at the end of 2012 via the social network.
"I'm lost for words," said Spies in the Yamaha statement. "We've had such a tough season with an unbelievable amount of bad luck. We have always gone out there to give it 100% and it was the intention of both myself and my team to continue to do so at Phillip Island.
"I took a really hard knock on Sunday and it's now clear from today's tests that I have some injuries that require immediate attention.
"It's disappointing to be heading back to the USA now instead of to Phillip Island but it's obviously important to get the right treatment as soon as possible.
"I'm sorry for my team and thankful for all the hard work they and Yamaha have put in so far; our current situation is not where we should be".
There was no indication of whether Spies is likely to be fit for the Valencia season finale, his final appearance for Yamaha before joining Pramac Ducati, which takes place from November 9-11.
It is assumed that Spies, currently tenth in the standings after a nightmare season, intends to make his Desmosedici debut in the post-race Valencia test.