"Polanco was diagnosed with a concussion," Miami manager Mike Redmond said. "He's going to go on the seven-day DL. We were, obviously, hoping that wasn't the case. But the next day he realized it was a little bit more severe, and he did have a concussion."
Replacing Polanco is Gil Velazquez, who had his contract selected from Triple-A New Orleans. To free up 40-man roster space, Marcell Ozuna was transferred to the 60-day disabled list.
"Gil has been around a long time, and he can do a lot of different things," Redmond said. "The biggest thing is we do not have to play a man short for a couple of more days."
In the eighth inning on Friday night against the Giants, Polanco was plunked on the left side of his helmet by Santiago Casilla's 94-mph fastball.
On Saturday, Polanco was upbeat and said he was feeling substantially better. He didn't have a headache, but he admitted to feeling a little dizzy.
It's too early to tell if Polanco will need more time than a week.
"It's hard to tell," Redmond said. "When you're dealing with concussions, those things are so serious. It's a matter of waiting and seeing. We will see how he feels the next five or six days."
Polanco, 38, has split playing time at third base with Ed Lucas.
In 91 games, Polanco is batting .253.
Velazquez, 32, is in his second stint with the Marlins. A year ago, the utility infielder appeared in 19 games with Miami in the second half.
Velazquez was batting .275 in 67 games with New Orleans.
Ozuna is out for the season after undergoing surgery to his left thumb.