"It's actually all lower leg, heel, feet, both of them now," Melvin said Friday. "He's having some trouble."
Blanks was sent to Triple-A Sacramento on Aug. 9 once he could run the bases comfortably, and he is 9-for-21 (.429) with a home run in seven rehab games. He appeared to be nearing a return after playing Monday and Tuesday, but he has not seen any action since.
The right-handed first baseman has been on the disabled list since June 23. In 45 at-bats for the A's this season, he is batting .333 with two home runs.
Oakland acquired Blanks from the Padres on May 15 in exchange for outfielder Jake Goebbert and a player to be named.
Jane Lee is a reporter for MLB.com. Aaron Leibowitz is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.