"It's been on and off for a bit," said Melvin, who noted it mostly affects Crisp when he's batting right-handed.
Melvin is likely to rest Crisp for Monday's series finale along with catcher Derek Norris, who is dealing with back spasms.
"There are a number of issues we're battling at this point," he said. "We'll just fight through it tomorrow."
Jane Lee is a reporter for MLB.com. Read her blog, Major Lee-ague, and follow her on Twitter @JaneMLB. This story was not subject to the approval of Major League Baseball or its clubs.