Nathan Thompson insists Pompey have to take maximum points from their final three matches.
They are currently sitting a single point outside the top six with games against Charlton, Bury and Peterborough remaining.
A midweek loss at Bradford ended a six-match unbeaten run and means the Blues’ fate is not entirely in their own hands.
But whatever happens elsewhere, Thompson knows that victory over the fifth-placed Addicks at Fratton Park on Saturday is a must.
The defender said: “I think the last three games were always going to be ‘must-win’ and Tuesday’s result makes that even more true.
“It was frustrating at Bradford because of our previous results and the position we’d got in. We let ourselves down a bit.
"It doesn’t take much motivation to get up for games like this – look at what happened against Wigan."
“I’ve said before that I think a club this size should be challenging for the play-offs, especially with the momentum from last season.
“But we had a sticky patch at the turn of the year and started to fall away a bit, so it’s not bad that we’re still within touching distance.
“It doesn’t take much motivation to get up for games like this – look at what happened against Wigan in the last home match.”
Pompey have already secured two 1-0 victories over Charlton this season, beating them at The Valley in both the league and Checkatrade Trophy.
Thompson said: “They’re in a similar situation to us and I’m sure those results will prey on their minds a bit.
“Sometimes when you’re at home and the opposition’s coming at you a bit more, it can open the game up.
“That could play into our hands because we’ve got a lot of speed on the counter-attack. It might suit us.”