The Saints placed the second-round draft pick on the injured reserved list, reports NFL Network's Adam Schefter.
Head coach Sean Payton discussed the injury earlier in the day but did not know at the time how long Porter would be sidelined.
"I don't want to speculate, but I know the procedure went well," Payton said. "It required surgery to get it back in place, which is not good."
Porter was the Saints' second-round draft pick last spring and won the starting job in training camp. Through the first five games of the season, he's been credited with 25 tackles, including a sack, to go with one interception and five passes defended.
Information from The Associated Press was used in this report