This.
I also wonder if this affects Poku. When Mix is in for Pirlo, you no longer need as much defensive cover. So does Mikey come out? Does that reopen the spot for Poku? Does a second season with this much bench time light a fire under Poku to play harder D?
This is an interesting thought. Maybe Vieira has grouped his guys into "lines" like hockey. First line is Bravo-Pirlo-Lopez with McNamara on the wing. Second line is Mix-Poku-McNamara with Mendoza/Taylor/Harrison on the wing. We haven't seen the second "line" recently because we've had a paced out schedule, but with Bravo out and three games in a week, maybe the second line will start @ Portland, back to the first line in Toronto and against the Red Bulls.
Probably not that cut and dry, but I could see these groupings as a theme of the lineup construction.