NBC's Michele Tafoya reported the injury happened as a result of a lingering groin injury the defensive back has already been dealing with for some time now.
In Greer's place, the Saints summoned backup defensive back Johnny Patrick, a 5'11" cornerback out of Louisville. The 24-year-old will be asked to cover one of the Chargers' top receivers for the remainder of the game and could see his role grow over the next few weeks, but that will greatly depend on Greer's availability.
Greer's absence will greatly impact the New Orleans Saints, who are currently working on a comeback at home against the San Diego Chargers, in the short term as well as the long term. The Saints have already struggled this year against the air attack, so losing a starting cornerback in Greer will be no help whatsoever.