The problem then is who do you play at 3? Move Root up and you risk losing the early momentum which is has played such a big part of our success, Morgan might be ok but he seems more comfortable in the middle order, Buttler is a definite no for me because he's very possibly the best 'finisher' in world cricket so we don't want him in too early. That leaves moving Mo or Stokes himself up to open (in place of Bairstow I'd expect given Roy and Hales have been excellent for us for the last 2 years).
That there's no obvious candidate is why he'd be the sub fielder for me right now.