The German joined the Blues in 2013 and although there were fleeting moments of brilliance, he never totally settled in London and eventually left for Wolfsburg 18 months later.
At the age of just 29, he recently announced his decision to retire.
Recalling his relationship with Mourinho, Schurrle said the current Tottenham boss could be savage with his words at times.
“He's a brutal guy,” Schurrle told Joko Winterscheidt in a YouTube interview.
“I always thought to myself: What does he do anyway? Why does he treat me like this? Why does he do this to people?
“In retrospect, I realise what he wanted and what resources he was working with. At the time, I couldn't really deal with the things he wanted from me because of all the harshness and the psychological pressure.
“Back then, it was extremely difficult. I would often drive home after conversations with him and just thought I couldn't do it anymore. What could I do? He was building up such extreme pressure.”
Schurrle first became acquainted with Mourinho during Michael Ballack's farewell retirement match in June of 2013 when he was in negotiations to move to Chelsea.
Mourinho, who was Schurrle's manager in the friendly, told the player he would loan him to Southampton if he didn't score two goals.
“At that point in time [June 2013] I was already in negotiations with Chelsea and Mourinho was my coach for the game. He said to me, 'If you don't score two goals today, I'll loan you to Southampton'.
"I didn't know if he was joking or not.”