Gallas spent five years at the Blues from 2001 - 2006 and won the Premier League twice.
But he eventually left Stamford Bridge for Arsenal in a deal that saw Ashley Cole head the other way.
The former defender says he clashed with Mourinho.
But he claims the relationship really soured after Michael Ballack was given his favoured shirt number.
Gallas told Four Four Two magazine: “In retrospect, I think that we did not understand each other.
“It’s a great pity, as we had the same common goal: to win trophies.
“But things got even worse for me when my No. 13 shirt was given to [Michael] Ballack.
“I didn’t expect that to happen and that was the moment I felt I couldn’t stay at Chelsea any more.
“That number had meant a great deal to me.
“I’d chose it because I didn’t know in which defensive position I’d play, and I liked it because I’d been told it wasn’t really a number that had been worn by an out-field player at Chelsea.
“I didn’t know anything about it being an unlucky number, but it suited me and became an important number for me while I was with the club.”
Gallas also went on to play for Tottenham after a four-year stint at the Emirates.
He hung up his boots in 2014 having spent the final year of his career at Australian side Perth Glory.
