The Inter boss believes the midfielder has huge potential but wants him to dominate a game for a full 90 minutes, while defending comments made by Mateo Kovacic earlier this week.
The Colombia international came close to leaving San Siro for Juventus in a swap deal with Bianconeri striker Mirko Vucinic in January, as his stock continues to rise.
Reports have also linked the midfielder with moves to Manchester United and Chelsea and Nerazzurri boss Mazzarri has acknowledged his burgeoning talent while highlighting the room for growth in his game.
He told reporters ahead of Inter's clash with Torino on Sunday: "Guarin is immensely talented. I've given him some advice to help him reach the top of his game. He mustn't go missing during the 90 minutes."
The former Napoli boss then moved to quash talk of a dispute between himself and Mateo Kovacic after the 19-year-old admitted his displeasure at only playing a bit-part role this season.
Mazzarri had previously hit out at the midfielder for his performance in the defeat to Juventus in February, but insists the pair remain on good terms.
"Kovacic's comments during the week? I watched the whole match when he played for Croatia the other day. I don't think he meant to say what got reported.
"I think he meant that he has to improve the defensive side of his game. He believes in what I tell him, I've spoken to him about it."