Cousins is starting Washington's third preseason game after Robert Griffin III suffered a head injury last week.
Cousins, drafted by Washington the same year as Griffin, has played in 14 games the past three seasons. In his career, he has completed 59% of his passes with 18 touchdowns and 19 interceptions.
Griffin has struggled to recapture the magic of his successful rookie season, when he posted a 102.4 passer rating. He struggled with knee issues in 2013, and last season, Griffin played in nine games but made only seven starts, slowed by an ankle injury and ineffectiveness.
Griffin has also reportedly had issues with head coach Jay Gruden. The No. 2 overall pick in 2012, Griffin has completed 63.9 percent of his passes with 40 touchdowns and 23 interceptions in his career.