The Boston Herald reported Thursday that Lester "possibly" had cancer, and was undergoing a series of medical tests.
Late Thursday, the team said Lester had been undergoing a series of tests to determine the cause of his recent back pains, and said several abnormally enlarged lymph nodes were identified.
Friday, the team said Lester's condition is known as anaplastic large cell lymphoma, and that he is expected to begin treatment in the coming week.
"Jon and his family wish to thank all those involved in his care at Massachusetts General Hospital," the family said through a statement. "Our gratitude also extends to the Red Sox organization, which has provided Jon and his family much needed support during this ordeal."
Lester is 7-2 with a 4.76 ERA in 15 starts this season.