The outfielder fouled a pitch from Mike Minor into the dirt, the ball deflecting off his foot and hitting the bill of his helmet and his eye. A trainer applied a towel and helped him off the field.
Reds manager David Bell said the foul ball hit Senzel above the eye, and that the swelling will be monitored before the club decides when Senzel will return.
"He's fine. He had a few stitches," Bell said after the Reds' 4-3 loss. "His eye is fine. ... Nothing serious, fortunately he closed his eye in time."
Senzel was the second overall pick in the 2016 Major League Baseball draft out of Tennessee, getting a $6.2 million bonus. Bouts of vertigo and a broken right index finger slowed has ascent. He sprained his right ankle during spring training and made his debut on May 3.
Senzel hit three homers in his first four games, a franchise record.
Information from The Associated Press was included in this report.