Forward Reus has not featured since the 1-0 defeat to Bayern Munich earlier this month because of adductor problems, while right-back Piszczek has been out since February due to an ankle injury.
The return of the pair will be a boost for a Dortmund side still pushing for a UEFA Europa League place at the end of a season that was confirmed last week as Klopp's last at Signal Iduna Park.
However, it is unclear whether either will be available for the meeting with Eintracht Frankfurt on Saturday, with Tuesday's DFB-Pokal semi-final with Bayern Munich seemingly a more realistic return date.
"If it's not enough for Saturday yet for Marco, it would be nice if he was available on Tuesday," Klopp said.
Meanwhile, midfielder Ilkay Gundogan has been forced to train away from the team due to a throat infection.
"Ilkay will train today, but not with the team," Klopp added. "He has a slight pharyngitis and we have to look, whether he's still infectious."
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