The 24-year-old, who made 10 appearances on loan at Derby last season, has been strongly linked with a return to Pride Park in recent weeks.
"At the end of the day, if Watford can't trade, we have to deal with that. It's not a failure on our part - it's just a fact of life," Rush told the Derby Telegraph.
"What we have to do is prepare ourselves accordingly and we will certainly be looking at our options in the coming days and weeks if we aren't able to secure Craig. It is a position that does need filling but I am very confident we will be able to do that."
Forsyth joined Watford from Dundee in 2011.