Bob Gunnell said Ali's doctors hoped to discharge him soon.
He added that "the Ali family continues to request privacy and appreciates all of the prayers and well wishes".
The 72-year-old former three-time heavyweight champion, who has Parkinson's disease, was taken to hospital on Saturday.
Mr Gunnell provided no further details.
Ali was diagnosed with Parkinson's in 1984, three years after his retirement from boxing.
He appeared in public at a ceremony in September in his hometown of Louisville for the Muhammad Ali Humanitarian Awards.
The condition of boxing legend Muhammad Ali has "vastly improved" since he was taken to hospital with a mild case of pneumonia, his spokesman has said.