I think Mendenhall is actually a pretty smart guy.
His off-season comments demonstrate an analytical mind which may serve him well in a debate, but it's an absolute killer for a running back.
The short answer is: He thinks too much.
A running back has maybe three or four tenths of a second to decide where he's going to hit the line but Mendy seems to need far more than that and by the time he's thought the whole thing through, there are five guys chasing him across the backfield and at that point he's lucky if he can just get back to the line of scrimmage.
Back in high school, they always told us that on a multiple choice test, your first answer is usually the right one.
Running with a football is a multiple choice test and he needs to go with his first answer instead of analyzing the question.
A "beautiful mind" may be an asset on an intellectual level, but for a running back it just spells trouble.
My advice to him would be: "Stop thinking and just run".