"I just don't know how serious it is," Gattis said Friday night after he was held out of the Astros' 16-7 win against the Toronto Blue Jays.
"We haven't announced anything, and I don't know what's going to happen if I feel better tomorrow, how fast or what the protocol [is] to start the process of getting back on the field."
The team has not yet placed Gattis on the seven-day concussion disabled list.
"Last night I just felt like I was recovering from something. I didn't feel terrible. I didn't feel great. But I knew I got hit in the head, you know?" Gattis said, according to the Houston Chronicle. "But I didn't feel that great today. I had a little bit of a headache and just a few symptoms. Moving forward I'm not sure what we're going to do tomorrow or if I feel good when I'll start doing baseball stuff."
Gattis is batting .278 with 11 home runs and 45 RBIs for the American League-leading Astros.
The Associated Press contributed to this report.