Feller, 92, has been receiving treatment at the Cleveland Clinic, Indians spokesman Bob DiBiasio told MLB.com.
DiBiasio said the former Indians great is getting stronger after coming down with a case of thrush -- an infection affecting the mucus membrane lining of the mouth and tongue.
Feller was diagnosed with acute myeloid leukemia in August, when he received two quarts of blood and began receiving chemotherapy treatments.
He later had a pacemaker installed to treat a heart ailment.
Inducted into the Hall of Fame in 1962, Feller pitched 18 seasons for the Indians, going 266-162 while setting the team record for victories. He was an eight-time all-star.
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