PSV are the current league leaders in the Netherlands with an eight-point lead over AZ Alkmaar, while Ajax sit in third spot.
De Jong scored a second half winner for PSV against Sparta Rotterdam on Sunday as they secured a 10th successive victory in the Dutch top flight.
The Eindhoven outfit head to Amsterdam next Sunday and De Jong insists they will not underestimate their opponents, as he feels both Ajax and AZ are still involved in the title battle.
“I think it will be a very tough game in Amsterdam,” he told PSV’s official website. “Ajax are still title contenders and AZ are having a good spell.”
Watford boss Javi Gracia does not think Abdoulaye Doucoure's form will be affected by his omission from France's latest squad.