You ask:
A future 20 ppg scorer 7 rpg but shooting .428% fg .300% 3p
Basketball-reference.com says:
There are only 18 such seasons in the entire 3-PT Shot era, achieved by just 12 different players. No one achieved that last season. The 12 players who have posted those numbers (> 20ppg, < .428 FG%, < .300 3PT%) while qualifying for the leaderboard are:
Russell Westbrook
Allen Iverson (4 times)
Jerry Stackhouse (4 times)
Paul Pierce
Michael Adams
Steve Francis
Tracy McGrady
Derrick Coleman
Lebron James
DeMar DeRozan
Michael Jordan
Latrell Sprewell
Not a bad list of names. In 14 of the 18 seasons, the player posted 4.0+ Win Shares and in 16 of those seasons, the player posted a PER of 17.4+.
It should be noted that most of these guys were actually 'scoring efficient' because most of them get to the free throw line at very high rates. In 15 of the 18 seasons, the player posted a FTr of .351 or better. The only rotation player on the Celtics to post that high of an FTr was Isaiah (.394). Jordan Mickey and Tyler Zeller also did, but they barely played. No other rotation player on the Celtics posted a rate above .314.
Judging by Brown's style of game in college and indeed, as he showed to extreme in Summer League, I don't think it's unreasonable to anticipate that he will be a player who gets to the FT line a lot. Like, at ridiculous rates. So even if he does, indeed, end up 'shooting inefficient' per your fears, I suspect he will actually prove to be very 'scoring efficient'.
Like the names on that list above.
I think most folks consider most of those guys to be, 'studs'.
Like the players named, it is possible that Brown might throw together such a season at some point, but I seriously doubt that he will really post such low percentages for his career though (while also scoring 20 points per game). No regular rotation player has ever come close to that combination of numbers on a career basis.