"It's been like that for awhile," Ramirez said prior to the opener of a three-game series with the Rangers. "I aggravated it against Jamie Moyer [Wednesday] night, and it was bad after that."
He's batting .166 overall and is hitless in his last three games but said he wasn't blaming the thumb for his struggles this season.
"I just haven't been consistent," said Ramirez, who hit his fifth career walk-off homer on Monday against Colorado.
Ramirez did pinch-hit in the ninth inning on Thursday against the Phillies but struck out swinging. His plan Friday was to get treatment and see how he feels Saturday.
"It's hard to swing the bat," he said.