Nicky Hayden was hampered by tendinitis in his wrist as the Ducati star set the eighth fastest time during the opening free practice sessions ahead of the Jerez MotoGP.
Hayden had been as high as sixth in the morning session but dropped two places in the afternoon as the problem with his wrist began to worsen.
He underwent a minor operation in March and although the pain from his wrist was 'manageable' in Austin, Hayden expects a much tougher weekend in Spain.
The American, whose best lap on the Desmosedici GP13 of 1m 40.722s left him as the second top Ducati rider on day one, 0.148s down on Andrea Iannone on the Energy T.I. Pramac Ducati in seventh, said he was also having a problem affecting the turning of the bike.
“We used a similar setup to what we had here at the test, with a few modifications that we've made in the last two races.
“In the morning, I felt a little better than I had in the test, and my pace was okay,” said Hayden.
“Then I suffered a lot on the warmer track this afternoon, especially with edge grip, which really affected the turning,” he added.
“The bike was pushing wide in the fast corners, which are so important.”
On his wrist problem, Hayden is adopting a grin and bare it approach.
“I'm also having a problem with my wrist; between Qatar and Austin, I developed some tendinitis and a lot of swelling in the joint,” he said.
“It was manageable in Austin, but it's worse here, which makes this weekend even more challenging than it already was.
“Still, if we can improve the bike in a couple of the key corners, we can be much closer.”