Watkins scored twice in the 3-1 quarterfinal first-leg win and was outstanding on the night.
Petrov believes Watkins can still make the final England World Cup squad, despite missing last week's 35-man selection.
He said on TNT Sports: "For Watkins, there were so many questions about his performances and scoring goals, but every time Aston Villa need him to produce, he does. That is what makes him first choice.
In the back of his mind he is thinking of the World Cup, definitely. When he is with England he has to score, he has to perform, tonight is a good start."
Former Chelsea winger Pat Nevin also said: "It's good timing isn't it? Lots of things happening at the end of this season.
"There is the possibility of the centre forward position waiting with the national team at the World Cup, there's a chance at securing Champions League football and there's a possible European final, and here he is scoring two goals and looking very sharp and very lively indeed. The chances he got, he scored.