The Argentine striker suffered the injury in the 2-1 win at Bristol City ahead of the international break.
Luhukay, who confirmed that Barry Bannan and Steven Fletcher will be available for Friday night’s televised game with fourth-placed Middlesbrough at Hillsborough - who Wednesday can draw level on points with if they win - said: “Fernando has an injury and is not available for the next few weeks.
“He got injured in the last minutes at Bristol with a hamstring problem and last week, he had a scan and the result was that he will not be available for a few weeks.”
Meanwhile, Luhukay says that Sam Winnall could make a further step forward in his road back to the first-team fray by appearing for the under-23s at the weekend - following a long-term injury.
Joost van Aken is also progressing well and is likely to play for the under-23s in the next few weeks according to Luhukay, also without a number of other players including Marco Matias and Gary Hooper.