Rush said: "Will is best placed in Derby.
"There is a huge amount for him to achieve here and I don't feel commercially it is the right time for us to entertain any other discussions.
"You look at a variety of factors when you make a decision.
"Is it right for the player? Has he already maximised his contribution to the club - is there any more he can do for us? And is he at the height of his potential in income you can generate?
"The answer to all three is 'no'."
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