When Jesus asked: "Who is the faithful and discreet slave?" He was not pointing to a specific singular entity that will spring up in the future. He was asking a rhetorical question directed to all who would be appointed to feed the congregation in some way. He was essentially asking them: "Will you prove to be faithful in teaching the congregation wholesome things at the right time?" I think Jesus asked this question in the context of telling his followers to keep on the watch because those appointed to teach the congregation would have a pivotal role to play in helping others to keep on the Watch. This connects back to the householder parable that immediately precedes the FDS parable in Luke. The slave in the FDS parable is the householder in the thief-householder parable immediately before. The householder has to keep on the watch to secure his house against the unannounced thief. The slave does this by feeding the domestics the right food to help them keep on the watch.
So the way I see it Jesus was telling everyone to keep on the watch and then used the FDS parable to give some focused counsel directed to the teachers of the congregation telling them that they have a special role to play in this regard and they had better prove themselves faithful in doing it. That's the way I see it.