He will now serve a one-match ban and Whittingham said: "I was not sure if bookings in abandoned games counted but it turns out they do, which is annoying for us and the lad."
Wycombe coped better with the sodden conditions and took the lead on the half-hour when Billy Knott finished well from 20 yards after Pompey had failed to clear a corner.
Ricky Holmes' free-kick was flicked on by Bradley and Agyemang found the net with an improvised finish from 12 yards.
"I have no complaints with the referee's decision," added Whittingham. "The ball was not rolling properly on the pitch so it was a no brainer."