Nicky Maynard faces up to four months on the sidelines after the Bristol City striker underwent a knee operation.
The Championship club were initially optimistic that last season's leading goalscorer would be back from the injury within two months.
However, after having surgery on Wednesday, new Robins boss Keith Millen has confirmed the severity of Maynard's injury.
"It is a massive blow for us losing someone who scored that many goals last season," Millen told the club's official website.
"Everyone is gutted for Nicky and now he needs time to recover.
"In the meantime we need goals and we are actively looking to try to add to the players we already have."
Maynard's absence follows on from the setback encountered by Sam Vokes after the on-loan Wolves striker was ruled out for up to eight weeks after tearing his hamstring on his debut against Millwall.