Ben Spies will miss this weekend's Australian Grand Prix because he needs surgery on injuries sustained during the Malaysian GP last Sunday.
The Factory Yamaha rider suffered a shoulder separation, a cracked rib and bruising to the lung after falling in the wet race at Sepang.
“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”Ben Spies
The 28-year-old American will return to the US for treatment on Tuesday.
In a statement, Yamaha said that they will not replace Spies for Sunday's race "due to time constraints".
Spies, who is joining Pramac Ducati next season, said in the statement: "I'm lost for words. 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."
Texan Spies is 10th in the championship with 88 points. His team-mate, Spaniard Jorge Lorenzo, leads the standings with 330.