The Magpies put in a disjointed display at St Mary's, with Alan Pardew's return to the stands at the home of his former club failing to have any kind of impact.
Instead, the Newcastle boss - back from a stadium ban but now serving a touchline one - was forced to watch on as his side never got going and were fortunate not to be well beaten by the interval.
Santon was flung into the action at half-time, to little effect, and the Italian concedes that the players let down themselves, their manager and a disgruntled travelling support.
He said in the Evening Chronicle: "I understand the fans' reaction at the end. I went over to say thank you, like I always do, and they booed.
"But I understand that - they deserve more from us and they are right.
"They come every game, pay a lot of money and travel a long way to support us, they sing for us and they do not deserve this performance."
Santon added: "We weren't good enough. We looked like we played with no passion and that we just want the season to finish, but that isn't true.
"It could have been 8-0 or 9-0 but Rob Elliot made a few good saves. We didn't shoot at all. I don't know if their keeper even got his gloves dirty or not."
Defeat to Southampton, which lifted the Saints back above Newcastle into eighth spot, all but ended any faint hope the Magpies had of forcing their way into European contention, but Santon insists they must ensure that their campaign is not allowed to fizzle out.
He said: "It doesn't have to be like this. We have to put in good effort in every game until the end of the season.
"Even though we know we cannot get a place in Europe or any competition now, we have to still play our best - that's what we didn't do (at St Mary's)."
Santon hopes he will be given the opportunity to help right a few wrongs, with the versatile full-back over the niggling knocks which have held him back at times this season.
The 23-year-old added: "I've had problems with my knee, my hamstring and then I have been ill, so it has been very frustrating.
"It was good to play 45 minutes, to get my feeling with the ball and the team back, but not good to be in a game like that."