Fister, who is competing with lefty Jason Vargas for the No. 5 spot in the starting rotation, only lasted two-thirds of an inning before Reds first baseman Joey Votto blasted a ball that hit Fister's arm and caromed to third baseman Jose Lopez, who threw to first baseman Casey Kotchman for the second out of the inning.
Fister covered third base on the play and didn't appear to be hindered by the blow, but after a brief visit from manager Don Wakamatsu and head trainer Rick Griffin, the pitcher walked off the mound and was replaced by Vargas.
Fister had the arm wrapped in ice during the game and saw a local doctor, who diagnosed a soft tissue bruise, "which is good news," Wakamatsu said. Fister will have a precautionary X-ray taken Saturday.
"There's discomfort a little bit, but it looks worse than it feels," Fister said. "There's a little baseball on my forearm."
Fister said he saw the ball, but didn't have enough time to react, and that he was taken out of the game for "just precautionary measures."
He added that the affected area was "very stiff, but that's it. No throbbing pain."
It was the second piece of painful news Friday regarding Mariners pitchers.
Earlier in the day, the team announced that projected No. 2 starter Cliff Lee has a right lower abdominal strain that was treated in Seattle with a platelet rich plasma injection and will be re-evaluated in a week.
Fister figured to be put into the rotation in the case of Lee opening the season on the disabled list.
Doug Miller is a national writer for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.