The Serb is in talks with AC Milan.
"Ljajic had been criticised in the past, but the fans are very close to him and would be happy if he stays," said Montella after a 4-1 friendly win over Gaziantepspor.
"Ljajic tells everyone he wants to stay, but that is not enough. He ought to remain with us for the sake of his growth process. I have already given him my advice, based on my experience."