If I had to give up either Tatum, Brown, or Irving for Leonard......
...give up Irving (injury prone) and roll the dice.
Play Tatum at the point (he will have to work on his dribbling) as his high IQ and underrated passing ability and size will make him a mismatch nightmare.
Hayward at the 2, Brown at the 3, and Leonard at the 4, Horford at the 5.
Height wise we'd be somewhat smallish but it's ok in the EAST but length wise we'd be pretty solid all across the board.