According to Ramirez's agent, Andy Mota, the strain occurred on Wednesday night. Ramirez was running the bases and fell on the shoulder while sliding. Mota said Ramirez was checked out as a "precaution" and that he should be fine.
Ramirez is expected to return to the Dominican on Saturday.
Ramirez, who turned 23 on Dec. 23, has been playing for the Licey Tigers in the Dominican. He will not be playing for the remainder of the Winter League season.
"We got word from the Dominican that he kind of tripped when he was sliding," Marlins general manager Larry Beinfest said. "He slid awkwardly on the left shoulder. We got the call yesterday morning. We didn't want to waste any time. It was precautionary and we also wanted to make sure it wasn't serious, and it doesn't appear to be so."
Based on the medical reports, Beinfest is confident the shoulder won't prevent Ramirez from showing up on time to Spring Training.
Acquired by the Marlins as part of a blockbuster trade with the Red Sox in November 2005, Ramirez became the Marlins' second Rookie of the Year winner. In 2003, Dontrelle Willis claimed the honor.
In 158 games, Ramirez batted .292 with 17 home runs and 59 RBIs. He added 185 hits, including 46 doubles and 11 triples, and he stole 51 bases.
Until Spring Training begins, Ramirez will be undergoing shoulder-strengthening drills.
Ramirez also said he is being shut down as a precaution.
"I feel soreness when I swing," Ramirez told ESPNdeportes.com. "It isn't something serious, but the Marlins and I prefer to avoid risks."
Ramirez appeared in just two games for Licey, and he was 2-for-9 with a run scored and a walk.
Ramirez tweaked the same shoulder in mid-May, when he swung through a changeup at Atlanta. He sat out a couple of days but never went on the disabled list.