A home defeat to Blackpool left Pompey seven points adrift of the League One automatic promotion places.
But the delayed start to the campaign means they still have 18 matches remaining to try to close the gap.
Charlie Daniels knows they need to do better than their performance against the Tangerines, who secured all three points courtesy of a late Jerry Yates strike.
And after successive losses for the Blues, the defender is looking for a reaction against Oxford in a few days.
He said: “Rasmus should have scored in the first half, although it was a great block by their guy on the line. If that goes in, it’s probably a different game.
“It’s a frustrating day and something we’ve got to learn from, making Tuesday that little bit more important.
"I don’t think it’s a mental thing, but we have to keep working hard in training and take our chances when they come."
“This is when you need the whole squad and it will take changes. There are guys on the bench or out of the squad who have to step up and take their chance.
“We can only control what we do, so have to look at ourselves and play better. If we do our job, then whatever happens, happens.
“I don’t think it’s a mental thing, but we have to keep working hard in training and take our chances when they come.
“You could see how just disappointed everyone in the side was when the referee blew that final whistle.
“Things need to change – and they need to change very quickly because games will start to run out for us.
“We have to start picking up points again and that’s got to start when we visit Oxford on Tuesday night.”