If you remember the Chennai team last year there was no one available for selection other than including Kedar Jadhav, the team did not give any replacement players either and Shane Watson was struggling throughout the tournament and the only option was to include an overseas player and there are limits for that as well and either they had to include Murali Vijay which is not another replacement in the T20 format.
Dhoni can only perform according to the team you get to lead and if you are getting a worse team you cannot perform well above his teams capabilities.
Dhoni was part of the CSK team which selected the players during auctions. Despite widespread criticism that the team is full of 35 plus players, he took no additional step to include fresh blood. There were no replacement for players who became unavailable. When Suresh Raina pulled out (due to some issues with the CSK management), the replacement for him was not available. Even this time, CSK went for players such as Robin Uthappa, who belongs to the 35 plus category.
Dhoni is the coolest in all situation. Along with this he have a winning luck with him. This is the reason why we can see more power performance from Chennai and at unexpected situations changing the entire match. Last year this was missing, but the team was criticised for its 35+ player list. In one of the season he played for Pune, by that time he took the team into playoff. So, more than performance he is a lucky player.