Hamilton was not in the lineup vs. the Brewers, as rookie Yorman Rodriguez started in center field, which manager Bryan Price said he had planned on doing anyway before the injury. Second baseman Brandon Phillips was moved up to the leadoff spot.
"He'll have to go through that whole concussion-testing protocol before he is cleared to play," Price said.
Hamilton said he expected to undergo testing later in the day. Players are required to pass the test, with the results being approved by a doctor with Major League Baseball.
With two outs in the top of the third inning during the Reds' 5-0 loss to the Brewers, Hamilton made a spectacular leaping catch at the center-field fence to rob Ryan Braun of a home run.
The right side of Hamilton's head hit the top of the wall, and despite the padding, he was holding his head as he came off the field. He batted in the bottom of the third and was replaced defensively in the top of the fifth by Chris Heisey after he reported feeling woozy.
In 152 games, the rookie outfielder is batting .250 with .292 on-base percentage, six home runs, 48 RBIs and 56 stolen bases. While it seems possible that the head injury would prevent him from playing again this season, Price was not ready to rule Hamilton out for the club's three remaining games vs. the Pirates.
"We wouldn't play him if there was any concern about his health," Price said. "I'd love it if he'd be able to play. He's a big part of our team. We still want to win as many games as we can through the end of the year, but we certainly wouldn't put him in any jeopardy if he shouldn't be playing."
Mark Sheldon is a reporter for MLB.com. Read his blog, Mark My Word, and follow him on Twitter @m_sheldon. This story was not subject to the approval of Major League Baseball or its clubs.