The 25-year-old Argentinian, who signed from Juve over the summer, had started to impress with the Hornets before being forced off in the first half of Watford's 2-0 loss to Manchester City on Wednesday night.
Speaking the day after that defeat, Mazzarri revealed that while he was still in the dark over the exact nature of Pereyra's injury, he is braced to be without him for the foreseeable future.
"We still have to wait for some evaluations of the MRI and scans," the 55-year-old Italian said
"Then we will be able to give an answer, probably after the next game."
Asked if he expected his absence to be a short one or a long one, Mazzarri replied: "For sure it won't be too short
"From my experience I have a feeling that it's not a terrible, terrible injury, but at the same time I think it's not (a) very short (absence) as an injury."
That will certainly rule him out of Saturday's trip to Sunderland when 11th-placed Watford will look to secure a result that could see them sitting in the top half at Christmas
Troy Deeney is expected to be recalled to the Hornets' starting line-up after reverting to the bench against City, with Mazzarri having one eye on the Black Cats and another on the striker's disciplinary record given he is one caution away from a ban
The fixture will also give Deeney another chance to grab his 100th Watford goal, a landmark that has eluded him in the nine games since he scored his 99th at the start of the October
"These are moments that can happen for a forward to have in his career," Mazzarri said of his captain's drought.
"He was unlucky last week because he had chances to score and didn't
These are moments that for a striker can happen.
"Maybe he's feeling the pressure of these 100 goals but for us it's just important he plays for the team and then scores
I'm not sure if it will be good for him to have had a rest or not, we will see."
Without Pereyra, and with Deeney's form slumping, the Hornets will again look to midfielder Etienne Capoue to provide a spark in an advanced role he has flourished in under Mazzarri.
The former Tottenham midfielder had never been deployed so far forward until the Italian came in and his boss drew parallels between the Frenchman and Marek Hamsik, the Slovakian whom he managed at Napoli.
"I think Capoue is a player that is in the position where he needs to start from behind and always comes to the fore because of the physique he has and for the type of player he is," Mazzarri noted.
"He reminds me of Hamsik as a player, I think he's very similar, a player that comes from behind and goes to score as well
He's got a good shot, good qualities to play in that position."
Source : PA