The Bayer Leverkusen sporting director has not been impressed by the Chile international's performances so far and says he needs to toughen up
Voller has not been impressed by the 24-year-old's showings so far, following his €3.5m move from Universidad de Chile in the close season.
"Of course it's so far gone very badly for him here," the 1993 Champions League winner told Bild. "When he started in the Europa League, the post and crossbar were more his opponents.
"Junior has not made the leap yet, unfortunately," said the 52-year-old, adding that a sale of the player was "not yet planned".
"I hope that Junior can still display to us the qualities he showed in Chile," he added. "But the lad must be a lot more aggressive from next season."
Fernandes, who is under contract with the Werkself until June 2017, has so far played a total of 63 minutes in the Bundesliga, scoring no goals.