The Czech Republic international will enjoy his 10th season at the club but warns the return of the Portuguese is no certainty of success for the Blues in the coming campaign
'The Portuguese's return to Stamford Bridge comes at a time when Manchester United and bitter rivals City have also changed their managers.
And Cech believes the impetus David Moyes and Manuel Pellegrini will bring negates any advantage Mourinho's return will give, and predicts the race for the Premier League title will be closer than ever.
"We would like to do as best as we can and we will work hard to achieve our goals," he told reporters.
"But it's very hard to see who will be the biggest force before the season starts.
"All the teams you have spoken about have great squads, they have kept those players together and improved by buying some players.
"Even if they have new managers, they have the same players. They will have new motivation under a new manager, so it will be difficult."