The Blues have only been beaten once in the Premier League since Mourinho's return to Stamford Bridge, and lie second in the table having won five of their first eight matches.
And Bertrand, who deputised for the injured Ashley Cole at left-back during Saturday's 4-1 win over Cardiff, insists this impressive record is no coincidence.
“The old Mourinho spirit is back, definitely,” he told reporters. “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 reinstilled 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.
“And it’s good to see that passion from the manager, too. It inspires the team.
“If you looked around [against Cardiff], he had the whole squad behind him, the fans came into it, and it was good support.”