The two sides will meet at Stamford Bridge for the first leg of their Champions League last-16 tie on Tuesday.
"Robert at the moment is playing in the form of his life, he's top-fit and full of confidence," said Flick.
"He has a real run at the moment, and we hope that he will continue that here in the last 16 against Chelsea.
"As a leader up front he is very important for us, but he can also show that he can put the opponent under pressure. It's very important he is positive.
"At the moment he is absolutely brilliant, but he's also a team player tracking back and covering in defence. I don't know how many goals he's scored this season off-hand, but he's right up there."