I don't know that we need miracles in CF, just 2-3 WAR combined from the 3 players which Herrera would have done by himself with a few more games played last year. If Moniak makes it impossible for Herrera to get back the job, great. If Vierling can really play the position well defensively and take bats away from either of the LHBs, also great.
And yeah, you need one of 3B or SS to be better than an 8-9 hitter, if not both. Also not too much to ask, I don't think.