because most commenters here only seem to be reading the headline: according to the surgeon, the patient started heavily bleeding first, and as he was trying to find/stop the bleeding, that's when the mixup happened:

Shaknovsky’s deposition testimony described the chaos in the operating room after Bryan began bleeding extensively, causing his heart to stop. Medical staff performed chest compressions, and Shaknovsky attempted to find where the bleeding was coming from.

“I couldn’t tell the difference because I was so upset,” he said, referring to the organ he mistakenly identified.

“It was like a overflown sink that’s clogged up, and I am looking for a fork at the bottom, trying to feel and find the bleed, and I was not able to do so,” Shaknovsky said. He added: “After 20 minutes of struggling – desperately trying – to save his life, that’s when the wrong-site event took place.

As to why he didn't notice the obviously wrong size of the organ:

Despite a spleen typically being significantly smaller than a liver, Shaknovsky said he believed Bryan’s spleen was “double the size of what is normal” because of a mass on it. Beverly Bryan’s lawsuit, however, states that a medical examiner told her that her husband’s spleen was anatomically “nearly normal”, according to NBC.

edit: more context in this comment: https://lemmy.world/post/46739636/23694470