Portsmouth manager Kenny Jackett says his side showed naivety in their 3-1 defeat by Northampton on Tuesday.
Pompey's defeat means they are now 13th in the early-season League One table with only two victories in seven games.
Jackett told BBC Radio Solent: "We certainly could have won it. There was no lack of effort or application.
"But at times there was a naivety about us. The third goal sums us up and we are 3-1 down at a period when we are trying to get back into the match."
Portsmouth ended a run of six games without a win when they won 2-0 on the road at AFC Wimbledon last weekend, but slipped to their third league defeat of the season at Northampton.
Jackett was particularly aggrieved by some of his players' decisions to leave the field whilst the ball was in play.
He continued: "Matt Kennedy runs off to get a drink - you don't let sides, even for a few seconds, have a numerical advantage. It is unprofessional and not necessary.
"We then have other players running off to get treatment for a bloody nose. If the referee lets you go off that's no problem, or if you are in trouble you go down.
"You don't run off the pitch and leave us down to 10 men. There's a naivety about us, but on the plus-side there's an enthusiasm, a good attitude and no lack of ability."