Heitinga has been at the heart of defence for recent victories over Manchester City and Stoke, both achieved without conceding, just two months after being dropped after some inept displays at the back.
The Netherlands international even took to Twitter to apologise to fans after a poor performance against Aston Villa, and his manager has applauded Heitinga for his resilience in battling to win back a first-team spot.
"John is mentally very strong," Moyes said.
"He's played in a World Cup final and against all the top players, so I think that single-mindedness brought him through.
"He had some real bad periods in the season. In a way, he had to prove himself again. If it was anyone else it would have broken them but John's Dutch mentality got him through it."
Everton travel to Tottenham on Sunday hoping to continue their bid for a top-four finish.