Forsyth played 10 times for the Rams, mainly as a makeshift left-back, during a two-month loan spell at Pride Park towards the end of the season.
He was recalled by the Hornets ahead of their Championship play-off campaign.
After overcoming Leicester in a dramatic semi-final, Gianfranco Zola's team face Crystal Palace at Wembley on May 27 with a place in the Premier League up for grabs.
Rush is sure Derby will conclude a deal for Forsyth, who has a year remaining on his contract at Watford, after the play-off final.
He told the Derby Telegraph: "Craig Forsyth is a player who did extremely well here, a player who we would like to bring in and I am confident once Watford's play-off commitments are finished we will do that.
"I would expect him to be at Derby next season."