See http://www.thefreedictionary.com/hold, definition 8: "Hold" can mean to assert, affirm, or regard in a certain way. That is, if you say, "I hold Bob to be an honest man", that means that you believe he is honest or you consider him to be honest.
If something is "widely held", that means that many people affirm or believe it.
So to say that this person is "widely held to be a great basketball player" would mean that many people consider him to be a great basketball player.
Lebron James is a well-known basketball player. So "widely held to be the next LeBron James" means "many people believe that he will become as great a basketball player as LeBron James".
(Or maybe in context they're talking about some other aspect of LeBron James. I'm not a sports fan and I don't know anything about the man, so if he's also famous for his charitable work or singing ability or whatever, it's possible that that's what they're talking about. But from the fact that the paragraph is talking about basketball, I'd guess not.)