Bylsma says Niskanen is day to day. Didn't specify injury.
Niskanen is currently playing a bottom pairing role, which can be accounted for. Keep an eye out for who takes his place on PP2 should the offensive defender miss some playing time.
Sounds like Niskanen is questionable for the next Pens game. Through 59 games, he scored three goals and 17 points. If he can't play, look for either a roster move or for Brooks Orpik to draw in.