The 27-year-old left the Nou Camp for Stamford Bridge in June after three years with the Catalan club, although former club Arsenal had an option to re-sign their former captain.
However, the midfielder made the switch to the west London club, where the chance of working for Jose Mourinho also proved to be a significant draw.
"It was very important for me that the deal was done before the World Cup," Fabregas declared.
"I wanted my head free and to know what I was going to do next season and where I was going to be. I thought I was at the right age and I believed I wanted a change. I looked at all the offers I had and after speaking to the coach, I thought Chelsea was the best decision.