Whelan can play tall as well as small, and has better disposal than the loose Brown. Green and other midfield types have been going into the backline, and with their better skills I'm not so sure we need Brown's rebound.
CJ can provide that to an extent, and can play tighter from what I've seen, although I'm not sure he is a 'natural' defender. I believe he alluded to that in an interview somewhere.