Mazzarri's new-look team were beaten 4-0 by Valencia in New York last night, just three days after Chelsea defeated them 2-0 in Indianapolis.
He told the club's official website: "Game-wise, we took three steps back from the match against Chelsea. Valencia were faster, always first on the ball.
"Instead we played with heavy legs, but it's my fault, yesterday (Saturday) I made them work intensely, 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 form for the league. I repeat, either we choose brilliance here and we don't work, or we go down the road we chose, hoping all this work will pay off by the time the league comes."
Inter begin their Serie A campaign with a home match against Genoa on August 26.