Sources told ESPN1000's Bruce Levine that Ramirez could be lost for at least a month.
Ramirez was injured on a backhand dive in the third inning of Friday night's 3-2 loss to the Milwaukee Brewers and will go to Chicago for tests on Saturday. He'll likely be headed to the disabled list and was wearing a sling and holding a bottle of painkillers after the game.
"I felt a pop. I've done it before," said Ramirez, who hurt the same shoulder in 2000 in Pittsburgh and missed six weeks. "It's been nine years since I've been playing, it's just one of those things."
Ramirez made a nifty diving catch of Ryan Braun's sharply hit grounder, but rolled over and immediately clutched his upper left arm and winced. Braun was safe at first after shortstop Ryan Theriot had to retrieve the ball.
"Once you dislocate it when you dive or something, it could happen again, but I've played nine years, seems like I've dived 3,000 times like that and didn't feel anything," said Ramirez, who didn't have surgery when he previously dislocated it. "It's just one of those things that happened."
Braun said he didn't see Ramirez's injury while running to first, but watched the replays.
"It didn't look good, obviously I hope for a speedy recovery," Braun said. "He's a great player. Obviously it's a huge advantage for us when he's not in the lineup."
Cubs manager Lou Piniella said the training staff couldn't get the shoulder back into place on the field.
"It was out and the doctor put it back in here when he came into the clubhouse training room," Piniella said.
Earlier Friday, the Cubs traded reserve outfielder Joey Gathright to Baltimore for utility player Ryan Freel to bolster their infield depth. Freel may be called on as soon as Saturday when he arrives in Milwaukee.
Two weeks ago, Ramirez strained his left calf running the bases and missed six games. Ramirez is hitting .364 with four homers and 16 RBIs and had hits in five of his previous nine at-bats.
Information from the Associated Press was used in this report.