The 22-year-old left-hander was placed on the disabled list Monday with back pain, but tests revealed enlarged lymph nodes. The team said Lester has a treatable form of anaplastic large cell lymphoma.
Red Sox manager Terry Francona visited Lester on Thursday.
Before the diagnosis was announced, teammate Curt Schilling referred to the melanoma his wife dealt with several years ago.
"Having been in a situation where cancer was an operative word, it's an incredibly scary thing," Schilling said Thursday. "You're talking about a kid at the beginning of his life."
Lester is 7-2 with a 4.76 ERA and 60 strikeouts in 81 1-3 innings.Click here for more sports news from FOXSports.com