Whilst I fully expect Rooney to be deployed in midfield, I don't believe he's capable of doing so in a midfield-two. That puts too much defensive expectation upon him, whereas playing in a 433 gives him the license to contribute both in defence and attack. If we're lining up 4231, then Rooney is playing behind the striker.
As for the right-winger, I expect that could be on rotation. We won't have a totally fluid front three if Ibrahimovic comes in. He'll remain fairly central, Martial on the left, but Rashford could occupy the right hand side. As could several others - Memphis, Lingard, Januzaj, Valencia, Pereira or Embolo (if he arrives).
433 suits this team and is the most balanced and fluid. We have Schneiderlin, Schweinsteiger and Carrick who can all hold. If we can bring in a box-to-box midfielder then we could be in business.