Zarate criticised Bilic for playing him out of position during his time with the club, as reported by Sky Sports.
The 28-year-old had ended his two-year spell at Upton Park for Florence by posting a message thanking the club and its fans for their support during his time in East London.
But no sooner had he arrived in Italy than he was criticising everyone else for his lack of game time during his stay in East London.
After being unveiled at Fiorentina, Zarate told the Italian press: "I'm happy to be back in Italy with a great team, and to show how I can play," said Zarate as reported by Sky Sports.
"It's a good chance for me to get back on track. In England, it was good because English football is enjoyable, but I wasn't too comfortable with my role at the club and that's why I decided to come here.
"I prefer to play in a free, attacking role, one that allows me to roam."
'West Ham a mistake'
He went on to score a sublime last-gasp goal in a 2-1 victory over Carpi to keep the Fiore firmly on track for a Champions League place.
"West Ham was a mistake and I didn’t think it through. The Premier League is fun and obviously the wages are very different, so that swayed my decision at the time," Zarate said.
But now he has changed tack yet again in a private message to Dave Sullivan, the eldest son of Hammers co-owner David.
Dave junior had messaged Zarate to congratulate him on his goal and in response the striker backed his old side for big things.
The messages were then shared with Hammers fans by Dave junior on his Twitter account.
Zarate said: "Thanks Dave, I am sure this West Ham can go to the Champions League."
Is Champions League achievable?
Whether Zarate is right or not only time will tell but Bilic's side could do themselves the world of good if they beat Southampton on Saturday.
Three points at St Marys would put an eight point gap between the sixth placed Hammers and seventh placed Saints and will also put Bilic's side within three points of fourth-placed Arsenal.
Reel the Gunners in by the end of the season and the Hammers could well be meeting Zarate and his new Fiorentina teammates in the Champions League next season.