From the article linked earlier.
"Asked by Illini Inquirer for clarification on what “head trauma” meant, Underwood gave few details on the player's medical condition but said Curbelo does not have a concussion. Many who suffer concussions can experience neck pain days and weeks after the injury.
“It’s not [a concussion]. It’s not that,” he said. “We’ve ruled that out. I’m not going to get into anybody’s medical stuff, but we did rule that piece out. It’s something he’s frustrated with and we’ll just keep monitoring it and see how it keeps moving from here. "