The Blues turned to a familiar face over the summer in their search for a new boss.
Owner Roman Abramovich has struggled to fill the coaching void since allowing Mourinho to slip through the net in September 2007 - with several high-profile managers having come and gone.
Chelsea have decided to go back to a man who landed five major trophies during his first stint at Stamford Bridge - including back-to-back Premier League titles.
Mourinho has made a positive start to his second spell, guiding his troops to second place in the top flight table, and Bertrand claims there is now a belief within the camp that they can prevail in every game they play.
The Blues full-back, who deputised for the injured Ashley Cole in Saturday's 4-1 victory over Cardiff City which saw Mourinho sent to the stands for a second-half rant at the match officials, said: "The manager has shown his spirit and desire. He did not deserve to be sent off but it's good to see that passion.
"It definitely inspires the team. At home you need to make it your fortress, and you need to do everything you can to win.
"After he got sent off, if you looked around, he had the whole squad behind him and the fans.
"That Mourinho spirit is back now. That was one of the things that left Chelsea when he left.
"We never thought about losing. We were Chelsea and we are going to win. No matter how pretty, ugly, whatever, we knew we were going to win.
"That's something that has been instilled into us again."