Balotelli, who could not even make Italy's provisional squad for the European Championship in France, has scored just once in 20 league games during his loan spell at the Serie A club.
"In terms of his character, he could do much better. The fact he's not playing at Milan, that Liverpool have abandoned him, means that he is at a crossroads," Ventura, who was named as Antonio Conte's replacement last month, told Italian media.
"The few times that Balotelli has really wanted to be a footballer, he has shown his quality. I have no prejudice, but his football must do the talking."
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Sampdoria president Massimo Ferrero urged Balotelli to join the club earlier this month, saying that the club would be the perfect place for the player to revive his flailing career.
Ventura, who will be in charge up to and including the 2018 World Cup in Russia, also criticised Italy international Graziano Pelle for joining China's Shandong Luneng, saying the 31-year-old former Southampton player made a mistake by leaving England's top flight.
"Players need to have a desire to win, going to a lesser league doesn't help in that regard," the 68-year-old added.
"As for Pelle, apart from his missed penalty (in the Euro 2016 quarter-final defeat to Germany), he had a positive campaign. But China is far away. Let's see why he moved."
(Reporting by Ian Rodricks in Bengaluru; Editing by Amlan Chakraborty)
