"I was a little bit dizzy and my eyes were dark," Rosario said. "It was the first time that's happened. It wasn't really scary. We're athletic, and there are times when we lose a little hydration. It could be because you don't sleep well or rest, but it's nothing bad. I'm good.
"Nothing serious, but they tell me to drink more water."
Manager Walt Weiss called the issue "a little scary. He was lightheaded and had to go through some tests."
Rosario said Saturday's condition was not related to the flu that cost him 13 days in May. Interestingly, however, he is playing at a weight lower than when he came back from the flu.
When he was sick, Rosario dropped from 228 pounds to 218, and by the time he returned he was 219. Rosario said he wanted to see how well he played at a lighter weight.
On Sunday, he said he was 217 and he wants to stay in that area. He has a .234 batting average for the season, but has hit .269 with a home run and three doubles in his last seven games.
"I've been working hard in the cage," Rosario said. "My swing is pretty good. My power is so-so, but it's getting there. If I keep my swing and swing at the right pitch, the ball is going to go."
Rosario returned to Coors Field before Saturday's game was over and might have been available if the game had extended.
Thomas Harding is a reporter for MLB.com. Follow him on Twitter @harding_at_mlb. This story was not subject to the approval of Major League Baseball or its clubs.