"I know Robert from Lech Poznan," Malowiejski told the Daily Mail. "Three years ago he was just a striker who could score goals. Now he can drop into the Wayne Rooney position and start combinations. And his heading has improved. He's become a more complete player.
"Playing in a very good team, and being in the first XI, has been very important for his progress. If he went to, say, Barcelona and was on the bench, he cannot improve.
"But in Dortmund, with a great coach and team-mates, playing in the Champions League, he is very capable with great potential.
"Training and playing with good players every single day, that's the reason why he is so good now. He believes in himself even more now. And he always did."