An athletic rim protector would be good. DeAndre Jordan would be a great fit, he can block shots at the rim, rebound the ball and with our current emphasis on pace he would be able to run the court and get buckets in transition.
Out of the draft, I think Willie Cauley-Stein, Karl Towns and Myles Turner would fit the bill.
I would also be very happy with Marc Gasol, who is known as a good enough defender to win defensive player of the year in the past.
I think Monroe, could end up becoming a better defender and has already made strides towards that this year. I think his post scoring would really help to spread our offense out.