Guerrier, 34, had a 4.80 earned-run average in 34 games. He is in the final season of a three-year, $12-million contract.
Withrow, 24, a former first-round draft pick, pitched three games for the Dodgers earlier this season.
Manager Don Mattingly quickly made Withrow a late-game option and said his return to the minor leagues reflected the crunch of "a numbers game."
On Saturday, the Dodgers sent veteran Peter Moylan to the minor leagues and promoted rookie fireballer Juan Dominguez.