How good can this player be? Pretty good. A USA team once had a 5/5 A 18yo 6'5" PF, who turned out to have 56 skill points and 11 potential. But the same 6'5" PG could have high passing and handling that lead to the 5 rating, but poor inside skills, and only MVP potential, and such players are valuable but also nothing particularly special.
Around draft time, you often see several threads like this in the forums, asking the same question, "how good can player X be?" And the answer is almost always the same: it's hard to tell. The 5 ball rating and grade are based purely off salary. Salary is determined by the primary skills, but for high potential players, it's secondary skills that determine how good they can be. High secondaries save a lot of out-of-position training and can lead to very unique, very profitable builds, but there's no way to tell from the rating and grade if a player has high secondaries or not, since secondaries have almost no effect on salary. So, it's hard to tell for sure how good this player will be until we actually see his exact skills after the draft.