Manager Ned Yost said Monday before the Royals faced the Yankees that this was the plan all along if Mendoza struggled.
A 15-year veteran, Chen won 23 games as a starter for Kansas City the past two seasons. But Yost said it made sense to use Mendoza in the rotation when Chen was roughed up in spring training.
Mendoza, though, has faltered with a 2-5 record and 4.87 ERA.
Chen was 3-0 in 19 relief appearances with a 2.41 ERA this season. He will start Friday against Cleveland.