The club's on-going injury crisis has forced the Italian tactician to start Doucoure in the Hornets' last two matches against Tottenham and Stoke, the 24-year-old having previously only played six minutes of Premier League football this season.
The Frenchman look nervous on his first start in the 4-1 home loss to Spurs but produced a much-improved performance in the defeat to Stoke on Tuesday.
Mazzarri says Doucoure's showing at the bet365 Stadium was one of the only bright sparks in the 2-0 loss and expects him to continue on an upward trajectory.
He told the Watford Observer: "He was good and played a great game actually. He grew a lot compared to the previous game he played and there are some things he still needs to improve especially with the shooting.
"He has to shoot more but he's a young player that’s got a big future in front of him."