Forrester has been blighted by an ankle injury since making the move to Rovers from Brentford in the summer, making just two substitute appearances for Paul Dickov's side.
The 22-year-old has recovered from that problem and was close to a first-team return after some impressive performances in the reserves.
However, during a reserve fixture last week he suffered a tear to his quad and is set for another spell on the sidelines.
Doncaster physiotherapist Alex Dalton told the club's official website: "Harry has suffered a two-inch tear to his quad which will keep him out for between four and six weeks.
"This is an unfortunate setback as the ankle injury that has kept him out so far has now healed.
"I will be working with him to get a rehabilitation schedule together - this is another frustration for the lad when things were looking positive."