Lattimore, last season's NFL Defensive Rookie of the Year, is by far the Saints' No. 1 cornerback. And the Saints already are without veteran nickel cornerback Patrick Robinson, after he suffered a season-ending ankle injury in Week 3.
Lattimore actually collided with teammate A.J. Klein during the Washington Redskins' second drive of the game. Lattimore was assisted by trainers on the field before being evaluated both on the sideline and in the locker room. He ultimately was declared out.
The Saints' pass defense struggled mightily early this season, ranking 30th in the NFL by allowing 311 passing yards per game heading into Monday night's contest. However, the pass defense showed improvement in last week's win over the New York Giants. Veteran backup/nickel corner P.J. Williams moved outside to replace Lattimore on Monday night, and safety Vonn Bell moved into the slot.
Lattimore also suffered a concussion last season and missed the following game.
The Saints have a bye in Week 6, which could give him extra time to recover.