Heynckes won the Champions League as coach of Real.
Asked why he thought Real were struggling this season, the veteran coach explained that their summer transfer business might have hurt them.
"They let three very good players go in the summer, in [Alvaro] Morata, Pepe and James, maybe to save on salaries," he said.
"They didn't spend big and they thought the youngsters could compensate.
"In reality, you need a good mix of youngsters and veterans."