Pietersen missed the Three Lions' series against New Zealand and the ICC Champions Trophy with a bruised knee, but in his comeback match for Surrey he scored 177 runs.
Flower told The Telegraph: "I am very happy he is back. It was great to see him get that score at Leeds. But it was also great to see how he was in the nets with us prior to that. He looked very confident and was moving well.
"He is very important to our side. He is a superstar with the bat who can dominate and intimidate opposition. He is a big physical presence and a big personality, and those people influence sporting contests."
The first Ashes Test against Australia begins on July 10 at Trent Bridge.