The Nationals announced the pitching change Sunday, less than nine hours before first pitch.
Scherzer was scheduled to take the mound Sunday for the Nationals in the tied series, but his next start was thrown into question after the ace came on in relief in Game 2 and struck out all three batters he faced in the team's 4-2 victory versus Los Angeles on Friday night.
Instead, Scherzer will start Game 4 on Monday, according to the team.
Sanchez went 11-8 with a 3.85 ERA in the regular season, his first with the Nationals. He has a 2-5 record and 3.12 ERA in eight previous playoff appearances for the Detroit Tigers and Atlanta Braves.
In Atlanta, Sanchez started one game during last year's NLDS between the Braves and Dodgers, allowing three runs in 4 2/3 innings. This season, he was 0-1 with a 3.18 ERA in two starts against Los Angeles.
The Nationals also announced that center fielder Victor Robles, who exited Game 2, underwent an MRI and was diagnosed with a mild hamstring strain. Michael Taylor will take his spot in the starting lineup, but Robles will be available to pinch hit in an emergency.