Jack described his father,who _____a brave boy many years ago,as a strong-willed man.

Jack described his father,who _____a brave boy many years ago,as a strong-willed man.
答案给的是must have been.答案是没错,的确合情合理说的过去.可是选项中还有must be 为什么不行?有人说must be是对现在的推测,可是从语法角度看填must be 没有任何错误,为什么不能填must be?句子中有many years ago存在,那么和must be用在一起,也是表示对过去的推测.