The veteran designated hitter hurt his right Achilles tendon during Boston's 5-1 victory over the White Sox Monday.
Ortiz was injured stepping on second base while trotting around the bases after a home run by Adrian Gonzalez.
"The MRI has shown there's not a tear, he's sore and he's going to get another opinion," Valentine said.
"I talked with some of the medical staff, they thought the 24-hour wait at least is something that gives you a different opinion sometimes. We'll wait at least the 24 hours and probably into tomorrow maybe until we get a second opinion."
Trainers fitted Ortiz with a walking boot as a precaution.