The Black Cats took a big step towards securing their Premier League status with a 1-0 win over Everton on Saturday, which lifted them six points clear of the relegation zone.
Cuellar told The Independent: "The manager has come in and made an instant impression.
"People are calling it a miracle transformation, but it isn't. It's a fresh mentality, he knows how to treat people, but it's down to hard work."
Sunderland are now 14th in the Premier League after picking up back-to-back wins under Di Canio.