After a relatively slow start to life under Mazzarri, who replaced Quique Sanchez Flores as manager during the summer, Watford have now won four of the last seven.
The Watford hierarchy wanted a manager who took a more disciplined approached to management when they were looking for Sanchez Flores' successor and Deeney admits Mazzarri runs a tight ship.
While Deeney feels the Italian has been helped by working with the best squad in Watford's history, the club captain admits the manager's no-nonsense approach is working.
“It’s tough,” said Deeney on the Hornets Show. “He [Mazzarri] demands every day. He’s a bit old school in the sense that he demands off everyone. If you get kicked in training, I’ve been with managers who send you in to go and get treated.
"He’s like, ‘No, you stay out.’ It’s the way it should be. He loses his temper as well. One of the criticisms of Quique last year was that he was a bit too nice, especially in the back end of last season, when he let people get away with things.
"Mr Mazzarri will not be doing that, is the easiest way to say it. He’s brought in a different way and he’s adapted, too. He’s not said we are playing 3-5-2 every game. He’s been flexible and [for example] moved Iggy to the left against Man United. He demands results.”