The Italian striker scored his first goals for the Hornets as his brace helped sink Everton 3-2 at Vicarage Road.
Okaka's introduction to life at Watford following a summer switch from Anderlecht was hindered by a hamstring injury, and he struggled on his first start for the club in last week's 3-1 defeat at West Brom.
But the 27-year-old tormented Everton throughout and came up with two well-taken goals, a stylish backheel and a near-post header either side of Sebastian Prodl's first of the season.
Mazzarri said: "It's true he was sharper against Everton but it also shows, which I've always been saying, that with many injuries unfortunately this is what happens.
"It takes time for a player when he comes back to recover and to be 100 per cent ready.
"We have already seen the difference between last week and this week with Okaka
He is a very important player and this also shows how important it is when you don't have these players available
"I think he is perfect for the Premier League."
Romelu Lukaku opened the scoring for Everton, but they were dead and buried by the time the Belgian added a late second
Ronald Koeman's side have won just once in 11 matches and face visits from title challengers Arsenal and Liverpool over the next week
Koeman admitted: "I am worried at the moment because we have had a bad run of results
"Maybe it's good we play them at home because we are strong at home, away we have more problems
Of course they are good teams - but we can beat them."
Source : PA