Mazzarri, who was appointed the new coach of the Nerazzurri in May following Andrea Stramaccioni's departure, has led the club to three games without a victory in pre-season, including on Sunday against los Che.
Two goals from Jonathan Viera and one apiece from Ever Banega and Jonas secured the win for the Liga side, but Mazzarri is confident that his team, who lost 2-0 to Chelsea in the pre-season tournament opener, will find form once the league campaign starts.
"Game wise, we took three steps back from the match against Chelsea," he admitted. "Valencia were faster, always first on the ball.
"Instead we played with heavy legs, but it's my fault, I made them work intensely on Saturday, but in this part of the summer preparations, I would rather want my team to be slow and work.
"We have to make a decision here: Either we want to be brilliant in these friendly matches - even though this is meant to be a tournament, to me it's still full of friendlies to build up your league form - or we go down the road we chose, hoping all this work will pay off by the time the league comes."
Mazzarri refused to draw comparisons between his former club Napoli and his new employers, while he believes Italian clubs' poor showings in pre-season, with Juventus losing 3-1 to LA Galaxy and the Partenopei suffering a 3-1 defeat to Porto, is because of the way Serie A clubs prepare for the campaign.
"Napoli is a closed chapter now. Last year I had a team I already knew, while here I have to create a system of play," he added.
"You can’t compare the two. Now we have to work on our new season and be ready when it starts.
"The fact that Juventus and Napoli have lost some pre-season friendlies? It’s probably because we coaches from the Italian league know what kind of preparation it requires.
"Our preparation is all based on work for the league, and the work will pay off."
Inter have one more friendly clash against Juventus before the Serie A season starts on August 25 versus Genoa.