Marshall was injured Tuesday night when he was struck by a ball hit by El Paso's Jason Hagerty in the sixth inning while pitching for Triple-A Reno. Marshall was able to walk off the field with help of teammates and had surgery to relieve pressure in his skull.
The Diamondbacks issued a statement Friday saying Marshall is alert and in stable condition.
The 25-year-old Marshall has appeared in 70 games for the Diamondbacks in parts of two seasons, going 4-6 with a 3.45 ERA. He was 0-2 with a 6.08 ERA before being sent down to Reno this season.