The 29-year-old is finally justifying Sir Alex Ferguson's decision to smash United's transfer record to land him.
And Petrov said: "Dimitar is on fire at the moment, but I am not surprised.
"I played with him for 10 years for our national team and I know what a great player he is. It's difficult to explain why it has taken him so long to show his true form for United.
"It has taken two years. Maybe they needed that time to see how he plays and so Dimitar could fit into United's style.
"He did very well at Tottenham and everyone expected Dimitar to carry on at United. With all due respect to Tottenham, they are not one of the biggest teams in the world like United and maybe Dimitar found it hard to adjust.
"Ferguson knew this and gave him time - and now he is showing how fantastic he is, scoring beautiful goals."