The 26-year-old has one year left on his contract, but Millen believes the forward may still put pen to paper on a new deal.
"I understand everyone's anxiety in him not signing a new contract and I'd love it to be sorted out sooner rather than later," he told the Bristol Evening Post.
"In my dreams, Nicky signs a new contract and gets the goals that win us promotion."
The club have already rejected three bids for the striker, which Millen said fell below the Bristol's valuation of the player.
The Robins begin their Championship campaign on August 6 at home to Ipswich Town.