Have you ever heard of rotation? Rooney and van Persie suffer enough injuries over the course of a season to offer sufficient gametime to our other attacking positions. If anyones place comes under jeopardy it should be Hernandez, as he has regressed, isn't happy on the bench and is unlikely to fetch a higher fee than his current market value.
A lot also hinges on which formation LvG decides to use. It isn't a certainty he will opt for the 4-3-3, as he could go for the 4-2-3-1 or 4-1-2-1-2. The latter would mean more space for both Rooney and van Persie, while the 4-3-3 and 4-2-3-1 means we are one long-term injury away from having either Hernandez or Welbeck lead the line for a long period of time. Severley underwhelming and not worth the risk given the sides other deficiencies.
I agree that three strikers should suffice, but Hernandez is the obvious player to discard out of out four current strikers. I just believe selling van Persie would prove to be a calamitous error.