Harrison left the game because of dizziness in the third inning. He will be tested with the Immediate Post-Concussion Assessment and Cognitive Testing (ImPACT) system.
"He did get dizzy in that inning, and he made the right move by letting people know and getting out," manager Clint Hurdle said. "We'll see how he feels through the evening and how he feels tomorrow."
In the bottom of the second, Harrison attempted to score from second on a single by Alex Presley, and collided with Astros catcher Carlos Corporan. He suffered a bloody nose and left the field holding a compress to his face.
He had made the third out, though, and returned to the field for the top of the third. In that inning, he made two errors -- the first, when he failed to field a ball hit by Hunter Pence, and the second, when he threw wide of first on a grounder by Matt Downs.
Harrison came to the plate in the bottom of the third and flied out to right field to end the inning, which is when he felt dizzy and left the game. Brandon Wood replaced him at third.
Shortstop Ronny Cedeno was placed on the 7-day disabled list for a concussion on Saturday. If Harrison were to also be diagnosed with a concussion and placed on the 7-day DL, he would be one of 11 injured Pirates.
Laura Myers is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.