Floyd Mayweather Jr. -- Jail is KILLING My Boxing Career

Floyd Mayweather, Jr. says he's begun to shrivel up into a puny regular person during his first 12 days in the big house ... and he's worried he won't be able to bounce back in time to save his boxing career.

Mayweather made the statements in new legal papers ... in which he begs the judge to release him from the Clark County Detention Center in Nevada and let him serve the remaining time in his 3-month sentence on house arrest. FYI -- Floyd is currently locked up for beating his baby mama in front of their kids last year.

In the docs, Mayweather says he spends 23 hours-per-day locked in isolation for his own safety. But he claims the imprisonment is taking a "cruel and inhuman" toll on his rippling physique because he can't train at a world class level while serving his time.

Mayweather claims his doctor is concerned about his "dehydrated appearance, his lack of muscle tone and his dry mucus membranes."

Floyd says his doctor also "expressed deep concern for [Floyd's] health and explained that any lengthy period of time with an inappropriate diet, coupled with lack of regular exercise, will most likely lead to irreversible damage to Mr. Mayweather's physique."

Floyd's doctor also told the court he's concerned about Floyd's diet ... because he used to eat 3,000 to 4,000 calories per day ... and now, the doc believes FM is eating less than 800.

In the papers, 35-year-old Floyd estimates he only has 2 years of boxing left in his career ... so every day counts.