The Frenchman fully supports the Portuguese's decision, saying the Spaniard was too weak for the Premier League and not as capable as Willian.
The Spain international was a popular figure with Blues supporters during his near three-year stint at Stamford Bridge, but failed to nail down a regular first-team following the return of 'The Special One' last summer.
Mata duly completed a switch to reigning Premier League champions Manchester United in a €45 million deal during the January transfer window and while Mourinho's decision to part company with such an asset may have raised eyebrows among some supporters, Leboeuf believes it was the right call.
"I've never been a big fan of Juan Mata," the Frenchman told Perform. "I think he's a fantastic player [but] I think he doesn't suit English football.
"I guess Chelsea tried to find the new [Gianfranco] Zola. But Zola was much more powerful than Mata, and Mata is a little bit light.
"I think Mourinho realized that and that is why he bought Willian, because he has a little bit of Zola, not as big as Zola but strong, skilful and works really hard for the team and can defend.
"With [Eden] Hazard, Oscar and Willian, it is an armada. You don't know where to look, because you know you are going to struggle at some point."