My point of view:
NO OFFSIDE.
The critical parts are:
Did Panucci fell due to foul? NO
Did he get injury that referee "called" him to leave the field? NO
Panucci didn't get any intention to leave the field to prevent offside
situation. That's why he didn't get yellow card in this case.
There would be Offside ages ago in this situation. But people may never notice it the rules have been changed.
Some critical decisions that can help us to know the rules more and let the players "wake up".
During in these 4 years, there'd be 3 critical incidents that every player will remember it and do want to break it again.
1) Tim Cahill was taking off 1/2 way of his jersey and got booked, as
he got his 2nd yellow card of that game, he got sent off (yellow for
unsporting behaviour)
2) Javier Mascherano got his yellow card as he was keep talking to
referee and questioning the decision of referee during first half vs
Man Utd. As he got his 2nd yellow card in that incident, he should be
sending off (yellow card for dissent word/action)
3) The goal was allowed by the European Cup, The Netherlands vs Italy,
when Panucci was out of field and he'd be letting Van Nistelrooy onside
in this situation.
I am asking some senior referees in my referee ass. and some of them
are giving me same answer.. they think there'd be a great decision from
Assistant Referee!