I don't get this line of thinking. Rooney starts whenever fit and rarely gets substituted, therefore opportunities for his back-up will be extremely limited unless Rooney gets a serious injury at some stage during the season, assuming we play 4-3-3 or 4-2-3-1. Personally I'd have sold Rooney a long time ago but I can't see any evidence to suggest LVG is even considering dropping Rooney. Then there is LVG's love for RVP; the rumours about him departing seem to be a whole lot of nothing as his wages are so out of line with his productivity right now.
The only way a new striker comes into play is if Rooney is used as a number 10, which creates all sorts of problems for the long list of attacking midfielders and wide forwards competing for playing time. If we sold Di Maria or Mata then LVG might just be able to keep the others reasonably happy in terms of mins on the pitch. Again, neither scenario looks likely at this stage, though PSG might still put in a serious offer for Di Maria.
Rooney/RVP/Wilson or Rooney/RVP/Hernandez or even Rooney/Hernandez/Wilson, are all realistic.
The likes of Lacazette, Kane or indeed Cavani, aren't going to be in a hurry to join United unless they are close to being a guaranteed starter; which really doesn't look viable while Rooney is such a dominant force at the club.