Do you have any data around rookies more often starting slow than fast? You're saying almost guaranteed, which to me suggests you think a rookie is going to start slow something like 80-90% of the time--is that supported, or is that simply a supposition on your part?
No, you didn't suggest sending him down; that part of my response was designed to show the danger in calling the first 3 games a "slow start". If AK's games 4 and 5 are identical to Buxton's games 2 and 3 from an offensive standpoint, we're (meaning everyone on this thread) not having this convo.
Miranda is in the minors because it's clear he would not play every day in MLB (Urshela and Arraez both need time at 3B); same with Larnach (Kiriloff and Sano need time at LF and 1B). If Kiriloff or Sano gets hurt, I would not be surprised if Larnach gets the call (unless he's still scuffling). With Miranda, it might take even more, as right now he's 6th on the infield depth chart (Correa, Polanco, Arraez, Urshela, Gordon), so it might take 2 out of those 5 getting hurt/struggling/being moved to the outfield for him to come up.