"I'd do anything for this team," he said.
Ramirez was set to bat third and play shortstop Monday night against the St. Louis Cardinals, and Andre Ethier was slated to hit fifth and play center field.
Ramirez, who sat out Game 2 in St. Louis, wore a lightweight flak jacket as he took swings during batting practice and fielded grounders before Game 3. The Cardinals entered with a 2-0 series lead.
"It's just for protection. They told me that if I want to play, I've got to wear it," Ramirez said. "They don't want anything worse to happen, so I'm just going to wear it."
The three-time All-Star said team medical director Stan Conte left it up to him whether to play.
"I'm not feeling 100 percent, but I feel good enough to go out there tonight and get our first win," Ramirez said. "I took some painkiller to make it feel batter."
Dodgers manager Don Mattingly said a CT scan revealed Ramirez has a hairline fracture of his ribcage.
"It's not dangerous if he tries to play," Mattingly said. "It's just a painful thing and it could be a week, it could be two weeks before this thing is gone."
Mattingly said it was a busy 48 hours trying to do everything possible to get Ramirez on the field, including talking to the NFL about using the kind of padded protective gear worn by running backs.
Ramirez said he couldn't sleep Sunday night.
"I know it's going to be a tough game," he said. "I'm going to keep everybody relaxed, and keep Mattingly relaxed. He needs to laugh."
Ramirez got hit by a pitch in Game 1, but stayed in for the entire 13 innings as the Dodgers lost 3-2. He missed a 1-0 loss in Game 2 on Saturday.
Ramirez batted .500 with six RBIs in the NL division series against Atlanta and tied the club's postseason record with six extra-base hits.
Ethier, put in the lineup following a batting practice evaluation, has been bothered by shin splints and was limited to pinch-hitting duty in Game 2. Ethier has a .300 career batting average against Cardinals starter Adam Wainwright.