Former Stoke City midfielder Eustace is thoroughly looking forward to his time at Pride Park, with the club currently 16th in the table after five wins in their last seven games.
He said on Sky Sports News: "Obviously I wasn't playing at Watford, and I want to be playing first-team football.
"Derby showed an interest and I jumped at the chance. (Manager) Nigel (Clough) seems to have turned it around. It looks an exciting place to be. There is a quality squad here.
"I've got good experience and hopefully that can help. I'm just raring to go. I am really excited to be here and am going to give 100 per cent."