Patel, 62, has pleaded not guilty to unlawfully killing pensioner Mervyn Morris, after removing his sigmoid colon in the Bundaberg hospital in 2003.
Dr William Renton-Power is a retired general surgeon in Rockhampton in central Queensland, who met Patel while lecturing in 2003.
Dr Renton-Power reviewed Mr Morris's medical records and told the court he believed the patient had diverticular bleeding, which is consistent with Patel's diagnosis.
He said Patel's decision to operate at the time was excellent and that he would have done the same.
Dr Renton-Power said there was a window of opportunity to operate while the patient was stable.
"You've come here to defend Dr Patel at any cost haven't you?" asked prosecutor Peter Davis.
"No," said Dr Renton-Power.
Mr Davis also accused Dr Renton-Power of assuming that Patel's colonoscopy was performed properly.
"I'm not assuming, I know," said Dr Renton-Power.
He is the fourth witness to back Patel's actions in treating Mr Morris.
Patel gives evidence
Patel returned to the witness box today and faced another barrage of questions from Mr Davis, who accused him of not performing the colonoscopy properly.
Patel disagreed, but did admit that he would not have had to carry out corrective surgery on Mr Morris if he had not performed the first operation.
Under re-examination, Patel again defended his decision to operate, saying the patient had a 50-50 chance of bleeding again without intervention.
Patel has now finished giving evidence.
The trial continues.