After taking eight points from their first four games in Serie A this term, Mazzarri's new-look Inter side have since crashed to comprehensive defeats against Cagliari and Fiorentina.
A 4-1 loss to Cagliari was followed by Sunday's 3-0 reversal at Fiorentina, leaving Mazzarri with work to do before Inter's visit of his former club Napoli on October 19.
Mazzarri feels a number of his squad were not physically fresh going into Sunday's fixture at the Stadio Artemio Franchi - something he needs to address before their clash with the Coppa Italia holders.
"I've already spoken to the lads because lots of them will be heading off on international duty now," the Italian explained.
"We just need to get our heads down and work on the problems we have.
"I've told them what I needed to say. We're all angry about the result and the way the match went. I'm confident we'll pick ourselves up again soon but we do need to work hard to solve our problems as quickly as possible.
"We thought certain problems could be overcome with a psychological shake-up but that proved not to be the case.
"I knew we weren't all that fresh. That's another reason for us to train really well over the next fortnight even if we'll be missing our international players."