Alright first off with Gallo I said if he came up and K'ed as much as he had in the Majors without a major burst of power it could hurt the confidence in his swing and might make him less effective.
With a pitcher though, it's ALL mental. If you get rocked like that after just coming from A ball it could make a real, permanent mental block on his ability to pitch.
It happens ALL the time.
The Mets would know if their own player could mentally handle a promotion to the bigs. And besides it wasn't even a big deal he was a temporary safety net for like two nights his development won't be scarred because of that.
Same goes for Gallo he's well aware of the flaws in his swing. At worst this call up is a just a step in his development process.