The Latics have not won since early August and another relegation battle appears inevitable.
Key players, including England striker Emile Heskey and skipper Mario Melchiot, are sidelined through injury and the pressure is increasing on manager Chris Hutchings.
Whelan said: "I was very disappointed with the effort and attitude of a few players last Saturday. It was a big letdown. Much better was expected.
"I simply cannot stand watching a team not giving 100 per cent.
"Wigan's trademark has been a never-say-die attitude and a desire to fight to the death.
"This has been lacking and I don't want to see a repeat of the Portsmouth performance.
"I speak with Chris Hutchings a couple of days after every game and I merely offer my opinions and views.
"I never interfere but we usually think on similar lines. That was the case on Saturday.
"The back four played well, and we were forced to go with a lone striker, but the midfield disappointed me.
"Emile Heskey has proved a huge loss, not only for us but for England as well. Not having him up front has been an immense blow.
"Saturday's performance wasn't good enough but we just have to battle on. And when our key players return, I'm convinced that we will see a dramatic upturn in form."