Mazzarri feels Guarin has been unsettled by his on-off move to Juve.
"If the transfer deadline is that late at night, it's only normal to consider the player won't be 100 per cent concentrated," said Mazzarri. "We have to consider all of this, especially in a big game against Juventus, so I knew I wouldn't call him up for this match.
"I don't think Guarin's sale in June is to be taken for granted. The club owners are sending signals, trying to do something for June, and from here to the end of the season I will evaluate all the players at my disposal."