Cisco hits well (so far) but not against lefties and everything else is "meh." Mountcastle hits well, doesn't walk and everything else is "meh". And Nunez? He's got nice power but he's looking like a .250 hitter with a .300 OBP type.
So yeah, I'd kick Davis to the curb and give Mancini first and figure out the rest.
You're arguing against a player saying he's got no additional significant tools...when the players you'd want to replace him with are equally one dimensional as well. To me, that doesn't make sense when you don't know if Sisco is for real and will ever be able to hit lefties, if Mountcastle can hit at the ML level and increase his walk rate and if Nunez could get his OBP to the .330 range. That's why I don't think they should trade Mancini. Because there's no clear cut better option and a lot more question marks.