The former Real Madrid coach came back in June after enjoying a successful spell at Stamford Bridge from 2004 to 2007.
With Chelsea flying high in the Premier League, Bertrand believes that the "Mourinho spirit" is now helping the club towards more silverware.
"The old Mourinho spirit is back, definitely," Goal.com quotes him as saying. "When he was here before, what filtered through to the youth levels was that we never thought about losing.
"We were Chelsea and we knew we were going to win, no matter how pretty or ugly we needed to be.
"That's something that has been reinstalled in us now, in the training, the preparation, the attention to detail, which give you the confidence to go out and do what you have got to do."
Chelsea return to action on Tuesday when they travel to Germany to face Schalke 04 in the Champions League.