As a result, Lester was admitted to Massachusetts General Hospital on Wednesday for further evaluation and testing, Red Sox medical director Dr. Thomas Gill said in a statement.
Red Sox manager Terry Francona said he talked with Lester and
family members Thursday morning but gave no further information
about the pitcher's health.
On Tuesday, the Red Sox had put the rookie on the 15-day disabled list retroactive to Aug. 24 with a back strain. He last pitched Aug. 23 in a 5-4 win against the Angels.
The left-hander is 7-2 in 15 games with 4.76 ERA and 60 strikeouts this season.
Information from the Associated Press was used in this report.