The Blues have lost seven of their 12 Premier League games thus far this season to sit 16th in the Premier League.
Both Hazard and Costa have been forced to endure alarming dips in form in the early stages of the season, scoring just two league goals between them, and Zola believes the duo’s poor performances are a key reason for the club’s current form.
“There is one compelling reason for the poor start to the season,” Zola told reporters. “The poor form of key offensive players Eden Hazard and Diego Costa has not helped.
“Defensively as well the team has dropped the standards set last year so the players have lost confidence as a group and this has led to poor results. The club has it in them to turn around their fortunes.”
Zola spent seven years at Chelsea as a player and currently manages Al-Arabi.