Add Eduardo Nunez to the Yankees’ ever-lengthening injury list.

In the fifth inning of yesterday’s 5-4 loss to the Athletics, the recently acquired Chris Nelson entered the game at third base, pushing Jayson Nix to shortstop and Nunez, who had started at short, to the bench. No immediate reason was given for Nunez’s departure from the game, as he appeared to be fine during the four innings he played.

However, after the game, the Yankees announced Nunez had a rib-cage injury, and an MRI exam taken at New York Presbyterian Hospital showed inflammation in the area.

“The MRI came back negative, so he’ll be day-to-day,” manager Joe Girardi said.

If his sudden departure from the game was mysterious, so was the fact the shortstop said he had no idea how he suffered the injury.

“I felt fine yesterday and when I woke up this morning. When I felt [the pain], I just thought it was something to do with my stomach, like indigestion,” Nunez said. “I told Joe what I was feeling and he asked me if I could keep playing. I said ‘Yes,’ but he decided to take me out to make sure everything was OK.”

Girardi said he was being cautious.

“You don’t want to make that a six-week injury if it can just be a couple of days,” he said.

Nunez is optimistic he will return to the lineup quickly.

“I’m happy [that] the MRI was negative and that there was just a little inflammation. I’ll be fine in a few days — hopefully [tomorrow], maybe. I’m going to get treatment [today], and maybe again in Colorado.”