Despite losing Harry Kane, Daniel Sturridge and Wayne Rooney to injury, head coach Gareth Southgate overlooked Deeney for England's upcoming matches against Germany and Lithuania.
Having scored five in his last six Premier League games for the Hornets, Deeney could have been forgiven for getting his hopes up with Mazzarri echoing the thoughts of many Watford fans over the snub.
"You should ask the England manager these questions," Mazzarri told reporters.
"All I can say is that for me he always plays and at this moment in time, Deeney is in his best form and scoring goals.
"I do everything I can to help my team, so of course I'll tell him from now on that he needs to score three goals in each game because one is not enough.
"Also, if he manages to score three goals then there is more of a chance that we can win."