Kevin Magnussen had to stop driving Haas' F1 car on Tuesday.
Ekstra Bladet newspaper reports that the Dane's head was pointing too far forwards, due to a problem with the headrest.
"I've been sitting really badly in the car," Magnussen confirmed.
"I had trouble seeing and when I braked, it wasn't easy to keep my head up. So I couldn't drive anymore."
However, Magnussen said a modification to his seating position will be "easy enough" for the American team.
Team boss Gunther Steiner confirmed: "It wouldn't have been safe to modify it quickly, so we'll get something done so he is comfortable for Thursday."
Explaining the decision to sideline Magnussen, the team boss added: "It just got too uncomfortable for him. He said 'My neck hurts, I'd rather stop now if you don't mind so it doesn't get any worse'."
Magnussen said it is too soon to say how competitive Haas will be in 2019.
"It's pretty easy to not get excited, because we now that everyone runs with different amounts of fuel, so the results aren't real," he said.
"I don't think we're bad, it's just difficult to judge.
"It's not the most exciting part of the testing," Magnussen added. "It will be more exciting next week, but there is no need for us to be worried."
