Sore arm. Poor training. Misjudging the distance.

COME ON FAUCI!!!

The Doc opened up about the debacle on the ESPN Daily podcast -- admitting he hadn't thrown a baseball in decades and wasn't properly prepared for the honor.

"I had been practicing for 2 days before at a walk down that I thought was 60 feet, but it was really probably 45 feet," Fauci said.

"I was throwing and throwing and throwing and I got it right but only it was at 45 feet!"

"And I kind of hurt my arm because I haven’t thrown a baseball literally in decades."

Fauci continued ... "So, as I was getting ready to go out on the mound, my arm was killing me -- but I said hell, I could suck it up and just throw it the way I had been practicing."

But, when Fauci got to the mound, the doc says he panicked -- "I said, 'Oh my god, Sean Doolittle looked like he was 200 feet away!"

Fauci says he changed his throwing motion to try and overcompensate for the distance and soreness ... but it backfired.

"I wound up and threw it like a bullet, only it just went way off to the left."

"So it was my bad, all the way."