The 21-year-old sustained the blow in a training ground accident and he is expected to be sidelined for up to a year.
Robins boss Danny Wilson admitted it was a serious blow to lose Amankwaah to such a long-term injury.
"Kevin originally hurt his knee about six weeks ago, but nothing serious showed up on the scan then, which is unusual with such an injury," Wilson told the club's official website.
"Last Friday he damaged the same knee in a training ground accident when turning and now we have learned the full extent of the damage.
"The main problem is with the anterior cruciate ligament, but there is also some cartilage damage. We have been advised that Kevin will be out for a year."
