Fernando Tatís Sr. explained the situation to MLB insider Héctor Gómez on Monday -- saying after the Padres outfielder got a chop recently ... he developed a fungus on his skin.

Tatís Sr. said his 23-year-old son turned to a medication to fix the issue -- but one of the ingredients in the remedy just so happened to trigger a positive test. Those results eventually led to the MLB hitting the slugger with an 80-game suspension last week.

Tatís Sr. wants it to be known that his son is definitely not a cheater -- and simply didn't look closely enough at the ingredient list in medicine for a skin condition.

"All of this has happened because of something that is not worth this issue," Tatís Sr. said. "This is something for the skin, that’s something that’s not performance enhancing and has no testosterone. It has nothing to help you improve in the game."

Tatís Sr. slammed baseball's decision to hand down the ban -- and said it's ultimately going to hurt the game more than the MLB realizes.

"It was a mistake that could have been handled differently," Tatís Sr. said. "Destroy the image of a player for such a small thing, for a situation like this is a catastrophe not just for Tatís Jr., but for all baseball."

"There’s millions of fans that will stop watching baseball."

For his part, Tatís Jr. took responsibility for it all -- explaining he had "no excuse for my error." He did also blame the test results on a skin condition (ringworm) and the medicine he took for it.

"I should have used the resources available to me in order to ensure that no banned substances were in what I took," he said. "I failed to do so."