The band got back together for one huge final show - the Back to the Beginning concert - at Villa Park in their home town of Birmingham, England, on July 5 and bassist Butler admitted he was shocked to see singer Osbourne in such poor health.
In an essay written for The Sunday Times newspaper, Butler said: "The rehearsals for that final show started a month before at a studio in the Oxfordshire countryside.
"Tony Iommi, Bill Ward and I ran through seven songs together. Of course, not having played together for 20 years, it took a couple of days to get rid of the rust.
"Then it was time for Ozzy to join us. I knew he wasn't in good health, but I wasn't prepared to see how frail he was.
"He was helped into the rehearsal room by two helpers and a nurse and was using a cane - being Ozzy, the cane was black and studded with gold and precious stones."
Butler went on to reveal his friend - who died on July 22 just two weeks after the concert - was unusually quiet during rehearsals because he found the work so exhausting.
"(Ozzy) didn't really say much beyond the usual greetings and when he sang, he sat in a chair," Butler said.
"We ran through the songs but we could see it was exhausting him after six or seven songs.
"We had a bit of a chat, but he was really quiet compared with the Ozzy of old."
Butler said he wished he had been able to spend more time with Osbourne because he had no idea the appearance at the gig would be the last time they would ever see each other.
"Nobody knew he'd be gone from us little more than two weeks after the final show, but I am so grateful we got to play one last time together in front of his beloved fans."
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