On Thursday, Ali's rep told ET that the boxer is in "fair condition."
"Ali, who suffers from Parkinson's disease, is being treated by his team of doctors and is in fair condition. A brief hospital stay is expected," the statement reads, adding that the family "respectfully requests privacy" during this time.
A source close to Ali told ET that he has actually been battling this respiratory issue for "some time." "He was invited to the Prince tribute held in Los Angeles a couple weeks back but could not make it because of this very issue," we're told.
Another source told ET that Ali has been slower to recover as of late from ongoing health problems.
"He's getting weaker and [it's] harder for him to recover from the chronic health issues," the source said.
Ali was previously hospitalized in January 2015 with a urinary tract infection, and again in December 2014 with pneumonia. He has been battling Parkinson's disease for over 30 years, since being diagnosed in 1984.
"I call my father in the mornings and that's when he's at his best," one of Ali's daughters, Hana, told ET in September 2014. "His voice is clearer, speaks slow but you can hear him. He has family around him and he's enjoying life."