He said: "I thought nothing of it at first but later it started aching. And when I took a shot I felt a sharp pain.
"I've got a lot of pain but we'll see how it goes over the next few days."
Spain's doctor Genaro Borras said: "Torres has bruising on the sciatic nerve in his right knee.
"We'll go easy with him but it shouldn't be anything to worry about."
Benitez is already at odds with England over the handling of his captain Steven Gerrard.