The Magpies never got going at St Mary's and were indebted to stand-in goalkeeper Rob Elliot for keeping them in the game early on, as he pulled off a string of fine saves.
There was only so long that he could keep the Saints at bay, though, and once the deadlock was broken on the stroke of half-time, the floodgates opened.
Southampton ended up running out convincing winners, with Pardew conceding afterwards that his visitors to the south coast had let themselves down with the insipid nature of their display.
He told Sky Sports: "We were second best today. We just didn't look like we had any energy at all for the game. Maybe the midweek game, I don't know, had something to do with that, but it definitely wasn't us today.
"We need to make sure this week that we do a lot of work and get ourselves ready for Man United, because that is a performance that isn't good enough for Newcastle.
"In terms of the energy that we had, it looked like they had more in every department. We lost the battle almost everywhere on the pitch.
"It was a really tough day and we didn't get any breaks in the game. All four goals are almost identical in terms of holding a line that wasn't there.
"We're desperate to freshen up a bit, but we're down on numbers and that's costly. But for all of our fans that have travelled down here, all this way and all the way back, I'm afraid that isn't good enough."
Pardew, who was in the stands after serving a stadium ban and starting a touchline one, was pleased to see Elliot shine in the absence of the injured Tim Krul, but believes he was left exposed on far too many occasions by those around him.
The Magpies manager added: "He did (play well), but he had far too much work to do.
"When you come here and you give the space to the quality that Southampton have got, you're going to be in trouble. We were in trouble from the kick-off to the end."