Ballack's first season at Stamford Bridge was a disappointment and his second is not going too well either.
The German international does not appear to be the same player who spent four years alongside Hargreaves at Bayern Munich.
"Michael Ballack was extremely successful in Munich," said Hargreaves. "He was Player of the Year every season he was there.
"For someone of his size, he has fantastic technical ability and his finishing, with both his head and his feet, is among the best in the world.
"But football is about balance and formations. Different players fit different environments.
"When Michael went there, Chelsea started playing a bit of a different system. They were still trying to find the right balance and he was still trying to adapt.
"But he is a top player and that does not change in one season. People have not seen the best of him by any means but, given an opportunity, I am sure they will."