Ali, 71, is in terminal decline from Parkinson's disease, Ali's brother Rahman told a British newspaper, The Sun.
"I know him better than anyone," said Rahman Ali, 69.
The older Ali, who called himself "The Greatest," was diagnosed with the degenerative disorder of the central nervous system in 1984. The disease is common to head trauma from activities such as boxing.
"My brother can't speak -- he doesn't recognize me. He's in a bad way. He's very sick," Rahman Ali told the newspaper.
"It could be months, it could be days. I don't know if he'll last the summer," the younger Ali said. "He's in God's hands. We hope he gently passes away.
"He told me before he got really bad that he's in no pain. He grabbed my arm and whispered: 'Rah, I've achieved everything I've ever wanted to accomplish. Don't cry for me, I'm in no pain.'"
But Muhammad Ali's wife, Lonnie Ali, posted a photo on Lockerz.com Sunday night of her husband wearing a jersey of Baltimore Ravens linebacker Ray Lewis. She said it was taken as Ali was getting ready to watch the Super Bowl.
The photo can be seen at http://tinyurl.com/readyforgame.
"All I can tell you is that Muhammad is rooting for the Ravens and Muhammad was completely [taken] with Beyonce," Lonnie told USA Today Sunday night.
Beyonce Knowles gave a dazzling Super Bowl halftime show Sunday that included a brief reunion with former Destiny's Child co-singers Kelly Rowland and Michelle Williams. The Ravens beat the San Francisco 49ers 34-31.
Muhammad Ali's poor health is "a rumor," Rasheda Ali-Walsh told USA Today Sunday night. "I just talked to him today and he's fine."
Family spokesman Bob Gunnell told The (Louisville) Courier-Journal: "He looks great. He's having a Super Bowl party."
Rahman Ali -- himself a former professional heavyweight boxer -- told The Sun he felt it was "best" his three-time world heavyweight champion brother die soon to avoid any "suffering and misery."
"He's going to heaven, there's no doubt," Rahman Ali said. "If his funeral was tomorrow, all the statesmen of the world would turn up. He touched everyone from the rich to the poor."
The Ali brothers -- born Cassius and Rudolph Clay -- grew up in the same bedroom of a two-bedroom home in Louisville with father Cassius Clay Sr. and mother Odessa Clay. They both later adopted Muslim names.
Rahman Ali, who has had 20 strokes, lives in a Louisville apartment with his minister wife Caroline.
Muhammad Ali lives in Scottsdale, Ariz., with Lonnie Ali.
Crowned "Sportsman of the Century" by Sports Illustrated, Muhammad Ali wants to be buried in Louisville and have a quote from 1960s civil rights leader Martin Luther King Jr. on his headstone, Rahman Ali told The Sun.
"This is his hometown. He'd want to be buried in the cemetery with Mom and Dad," Rahman Ali said. "On his tombstone, he said he wanted the Martin Luther King quote: 'I tried to love somebody. I did try to feed the hungry. I did try, in my life, to clothe those who were naked. I want you to say that I tried to love and serve humanity.'"
Below the quote would be "The Greatest," Rahman Ali said.