Patel, 62, has pleaded not guilty to unlawfully killing pensioner Mervyn Morris, who died three weeks after colon surgery in 2003.
It is the first time Patel has testified in defence of the allegations.
He told the court he remembers Mr Morris as a very pleasant, honourable man, who never complained of anything.
Patel said it was hard to forget someone with such unusual complications.
Earlier, Ken Fleming opened the defence case, telling the jury it was Patel's reasonable and honest opinion the surgery on Mr Morris was necessary and that he was fit to undergo it.
Mr Fleming added that doctors' opinions differ and sometimes despite the best efforts of a surgical team, patients unfortunately die.