Just to put some actual numbers on things, here's how the Astros outfield fell out last year:
Springer: appeared in 140 games/started 137, 85 cf, 78 rf
Reddick: 134/121, 48 lf, 11 cf, 102 rf
Aoki: 71/56, 56 lf, 16 rf
Gonzalez: 134/120, 47 lf, 2 rf
Marisnick: 106/65, 6 lf, 93 cf, 3 rf
Fisher: 53/39, 38 lf, 3 cf, 12 rf
Beltran: 129/118, 13 lf, 1 rf
I assume the appearances numbers are game appearances and not starts, but that should account for substantially all of the outfield starts for the year. I think from that it's clear that Marisnick will be the 2018 left fielder. The one thing most interesting to me is how many appearances Beltran made as a DH.