The 51-year-old Inter coach says that he has made it clear to the Colombia international that he must be on his game every week and compared Rodrigo Palacio to Edinson Cavani
The Colombia international has played a key role in the Nerazzurri's suprisingly strong start to the new season but the 27-year-old was off his game during the 2-1 victory over the Viola at San Siro.
Guarin was roundly whistled when he was replaced midway through the second half and Mazzarri has admitted that he was just as disappointed by the former Porto player's performance.
"Fredy knows he has to work [on his game]," the 51-year-old trainer told Radio Deejay. "He had a great game against Sassuolo [last weekend]. Then, against Fiorentina, he seemed dazed.
"He knows well that this must not happen. San Siro can have this effect on people; we know this. But still, it should not happen."
Mazzarri, though, has no complaints about the form of Argentine attacker Rodrigo Palacio, who the former Napoli boss believes can be Inter's answer to Edinson Cavani.
"He plays like Cavani did for me," the San Vincenzo native said of the Uruguayan. "In the modern game, the full-backs and the wingers cut inside, so one needs a striker with good movement.
"Palacio is doing this very well, just as he also does as a support striker."
Mazzarri spent four successful seasons in charge of the Partenopei before departing for the Giuseppe Meazza at the start of the summer.