Chapman was taken to the hospital and would be kept overnight for observation, the Reds said on their Twitter account.
Reds manager Bryan Price said Chapman was conscious and talking as he was taken off the field during their game against the Kansas City Royals.
"Not good," Price said. "He left the field on a stretcher, took a line drive just above his left eye is what it looks like -- a contusion, a laceration, and certainly needs to be taken to the hospital and checked. We've got Tomas Vera, an assistant trainer, is going to be with him. And then we'll get our updates from there."
The hard-throwing left-hander was struck by Salvador Perez's hit with two outs in the sixth inning. The pitcher crumbled to the ground, face down and flailing his legs. The ball caromed into the third base dugout. Medical personnel, including Royals Dr. Vincent Key, rushed the field.
Players from both teams huddled around the mound as the 26-year-old Cuban was being attended to and the stadium became silent. An ambulance's siren could be heard in background while Chapman was loaded onto the stretcher.
"I know this isn't uncommon as we would like it to be, but it was frightening, certainly frightening," Price said.
The game was then called with Kansas City leading 6-3.
Chapman, who throws a 100 mph fastball, had walked four Royals in the inning before being hit.
The two-time All-Star had 38 saves for the Reds last season.
In January, Major League Baseball approved protective cap for pitchers after the occurrence of several terrifying incidents similar to this one in the last few years.
The heavier and bigger new hats were available for testing during spring training on a voluntary basis but the cap apparently would not have helped Chapman in this case.
Oakland pitcher Brandon McCarthy, who had emergency surgery after he was struck in the head by a line drive in September 2012, urged caution in a Twitter post.