Johnson’s Senate office released the above photo of the senator in mid May.

WASHINGTON (CNN) — Sen. Tim Johnson will be able to resume his duties as a U.S. senator, his doctor said Monday in a statement issued by the Democrat from South Dakota.

"In my discussions with Sen. Johnson, I am very well aware that he is interested in continuing his work as a senator and I am confident that he will be able to resume his duties," said Dr. Michael Yochelson, director of brain injury programs at National Rehabilitation Hospital in Washington.

"During my last visit with him last week, he demonstrated physical improvement in his ability to walk across the room himself," he said. "I could also see continued improvement in his speech as it has become more fluid. With his improving language skills, the senator is able to express himself more clearly, which allows us to recognize the fact that he is doing well cognitively. He is reading the paper daily and talking with friends, family and colleagues. His memory and processing skills are strong."

Johnson's most recent MRI shows continued healing since last December's brain surgery, said Dr. Vivek Deshmukh, who performed the surgery.

Johnson receives therapy five days per week several hours per day, is working from home and is focused on returning to the Senate, the statement said.

Last December, Johnson, 60, began slurring during a talk with reporters. He was taken to George Washington University Hospital, where he underwent surgery to stop an intracerebral hemorrhage caused by a hereditary condition in which blood vessels are too close together — called an arteriovenous malformation.

His health had caused anxiety among Democrats in the Senate, where they hold a 51-49 majority. Had Johnson stepped down, his successor would have been named by South Dakota Gov. Mike Rounds, a Republican.

During his recovery, Johnson has been replaced by Sen. Barbara Boxer, D-California, as chairman of the Senate Ethics Committee.