I would argue that there's different views on your own squad and others. Every player in your own squad is key, for they can all make or break a game at times and all have their veins of extroardinary form. I would also argue that there are players more important to Man Utd than RVP and to a lesser extent Rooney.
Michael Carrick for instance is a lot more important when it comes to keeping the ball and using it well than both of those players, yet he recieves less public acclaim and is often slated in the public eye. Another player who was regarded with the outmost respect and admiration by the very same dressing room prior to his departure was Park. While not always a starter he was regarded as a very important player among the staff and players there.
While you cannot ignore players such as RVP and Rooney, the players you truly want and need to stop, should they play, is the player that keeps the team ticking over, and that is players such as Carrick and Scholes more so than RVP.
Take Swansea as another example, if you can stifle Britton/De Guzman/Ki/Hernandez, you have in effect halted Swansea in a more effective way than if you focus your attention on Michu and Dyer.
To summarise, the notion of key player is slightly different from a tactical point of view than that of the average pundit/fan as they too often focus solely on who was second last and last on the ball when a goal is scored. Whereas a manager often seek to stop the player who began the attack in the first place.
That is my intertpretation of the matter at least.