Pompey are now nine points away from safety with just four games remaining and Appleton believes that time is not on the side of the South Coast club.
"I'm realistic enough to know that we're going to have to win four games and unless we do we're going to be looking at playing in a different division," admitted Appleton.
"We started the game really well, created a few good opportunities, got in behind them, but the first goal in football is so important and we've allowed the centre-half to lump a ball up the field and given a really poor goal away.
"You could see in the second half the confidence oozing away from the players and it wasn't nice to see given the amount of effort they've put in over the last few weeks."
Portsmouth sit 23rd in the Championship and seem destined to play in English football's third tier for the first time since 1983.