Floyd Mayweather -- MOCKS Oscar De La Hoya's Substance Abuse Problem

Floyd Mayweather doesn't think Oscar De La Hoya's stint in rehab is off-limits when it comes to talking smack -- and today, the boxer took a BIG shot at the Golden Boy's substance abuse issues on Instagram.

Money posted a meme featuring a picture of Oscar looking sad ... with the caption, "Meanwhile In Rehab."

Another caption reads, "Oscar: I gave Canelo the wrong blueprint I was high."

The pic is an obvious reference to Floyd's big victory over Canelo Alvarez this weekend ... which took place one day after Oscar (Canelo's fight promoter) checked himself back into rehab for substance abuse.