"If you watched [the Home Run Derby], it didn't look like it was bothering him," Melvin said. "Usually those types of things happen on one particular swing, and it certainly didn't."
Cespedes spent four weeks on the disabled list last season because of a left hand injury. The exact nature of his current ailment remains unknown, and Melvin simply referred to it as "soreness."
"He feels better today, but until our training staff feels like he's 100 percent and not putting himself in a position where he could further injure it, it'll be day to day," Melvin said.
The 27-year-old has struggled a bit in his sophomore season with Oakland, posting a .225 average with an on-base percentage below .300. Cespedes has launched 15 home runs -- one behind Josh Donaldson and Brandon Moss for the team lead, but Sunday will mark the one-month anniversary of his most recent long ball.
AJ Cassavell is a reporter for MLB.com. Follow him on Twitter @ajcassavell. This story was not subject to the approval of Major League Baseball or its clubs.