The club confirmed the lengthy absence for the 21-year-old who suffered a cruciate knee ligament injury in their opening pre-season friendly against Parkgate FC.
The Millers’ record signing went down in agony in the 25th minute of the 7-0 victory.
The setback heightens Rotherham’s need for a new striker for the Championship campaign which begins in less than three weeks.
Clarke-Harris turns 22 on Thursday.