Casey Stoner will undergo further medical tests in America this week in a bid to understand the illness that has hampered him since the Catalunya Grand Prix.
At Laguna Seca he again fell back in the race as his physical condition worsened -- although this time he felt his slump from second to fourth was influenced by discomfort from his big qualifying crash and the after-effects of the winter surgery on his left wrist.
"Today I didn't feel as bad at the end of the race as I have done in the last two but I'm still not right," said Stoner. "I found it really difficult to keep my concentration over the last few laps and I was in a bit of a daze by the end.
"My left arm was feeling really tired by the end because of the fact there are so many left-handers here and my left wrist still doesn't have the full range of movement after my operation in the winter. I'm really disappointed because without all of these problems we could have done so much better.
"I want to thank the team for giving me a great bike and also Doctor Claudio Macchiagodena and our physiotherapist Freddie [Dente] for all their help. Now we'll focus on trying to find out the root of the problem because racing in this condition is not much fun."
Ducati team boss Livio Suppo said the additional problems facing Stoner at Laguna made it hard to judge how big a factor his original illness was.
"Surely yesterday didn't help to understand how he would be today without the crash," Suppo told Italia1. "He says he feels a lot better stomach-wise, as the sickness felt at Barcelona and Assen went a lot better.
"Despite that, especially in the last laps, he lacked concentration, and he told me he practically had to lean on the fuel tank under braking. His left arm felt very uncomfortable, so he wasn't at his best.
"Tomorrow we'll do a series of medical tests and we'll take advantage of being here to do them immediately instead of waiting to be back in Europe. Together with our doctors in Italy and with Clinica Mobile we've gathered all the things that need to be verified, and I think we'll understand well how to intervene. Doctor Macchiagodena has done a lot for Casey this weekend, we really need to thank him."