But they think they may have an idea.
"The days that I do wake up and maybe drink some coffee, those are definitely days I feel more jittery," Damon said. "We'll start with that, and maybe that's it. Hopefully that's it."
The Yankees' medical staff has instructed Damon to cut back on his caffeine intake to see if his eyes stop fluttering. He said he customarily drinks three cups of coffee or energy drinks in an eight-hour period to begin each day.
"With the way the game is, being an older player, I definitely need a few more energy drinks than the young kids," said Damon, who will turn 36 in November. "It could be a possibility. We'll see. In a couple days, we'll probably know more."
Damon said he hasn't had any caffeine for several days and woke up Saturday with a headache. He had not had any eye trouble so far, but said he hasn't had to focus on anything, either. Damon added his eyes have been sore, possibly from blinking too much lately -- "working overtime," he called it.
Damon also dismissed the possibility that this is the reason for his current hitting slump. In his past six games, he has gone 4-for-22 (.182) and seen his batting average dip to .285 -- his lowest clip since May 5. He was batting as high as .327 last month.
"I've been dealing with it for a couple weeks, so I feel confident in my abilities," Damon said. "If I don't get a hit or anything, it's my fault or the pitcher has done his job. I feel capable of playing, and every time I step into the box, I feel like I can do something."
Manager Joe Girardi said he spoke to the Yankees' training staff and was told Damon is available to play. Girardi said before Saturday's game that they are still waiting for the results of several tests.
On Friday, when Damon first announced his eye troubles, he expressed concern that he was suffering post-concussion symptoms, dating back to an injury he suffered in 2003 in a collision with Damian Jackson while a member of the Red Sox. He said Saturday, though, he is confident this is unrelated to the concussion.
"I was worried about that. It was definitely a vicious hit," Damon said. "I think everything's fine -- seems like all that post-concussion stuff stopped about two years ago. We're going to start with laying off the energy drinks and the caffeine and all that stuff, and hopefully the twitch will go away."