Warren Buffett's advice to LeBron James

The Oracle of Omaha had some financial advice for NBA superstar LeBron James on Monday.

"Through the rest of his career and beyond, in terms of earning power, [he should] just make monthly investments in the low-cost index fund," Buffett told CNBC's "Squawk Box" on Monday in response to a video question posed by James. "Somebody in his position ought to have a significant cash reserve."

Buffett said James should keep it simple when investing. "Athletes generally tend to get promoted by people with restaurants and real estate," Berkshire Hathaway's CEO said. "Everybody's got an idea for him and, usually, the simplest is the best."

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Buffett also said James should invest primarily in American companies. "Owning the United States at a decent average price bought over time, you really can't go wrong with that," he said.

Buffett also said someone like James should keep his investing strategy simple because of the demands that come with being a professional athlete. "Their expertise is making a lot of money doing something they do extremely well, but they aren't generally going to be able to take the time to become a professional investor."

James also asked Buffett for some basketball advice. Buffett said jokingly: "We went up for a jump ball one time and he got it, went the length of the court [and] dunked it just as I was starting to jump, so he does have that much to learn from me."

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