Benjamin Miller, 20 was gored multiple times receiving injures to his thigh, sphincter and back when he was tossed by a fighting bull during Saturday´s running of the bulls.
After three hours of surgery the American from Georgia was said to be "recovering well" in intensive care at a hospital in western Salamanca.
Enrique Crespo, the surgeon who operated on Miller, described the injuries, which included a wound measuring 40 centimeters, as "the biggest goring wound I´ve ever seen," according to a report on television channel Cuatro.
A spectator captured the moment the bull caught Miller and tossed him in the air on the horns. Onlookers screamed in the crowd while others rushed to distract the bull and attempt to pull Miller to safety.
Two other men, reported to be a Scot and a Spaniard were also injured during the encierro although their wounds were described by authorities as minor.