Sam Allardyce believes there is a fear factor among Sunderland's players after seeing the Black Cats fall four points adrift of safety.
Allardyce suffered a third defeat in his four-game reign after Dusan Tadic's second half penalty was sufficient for Southampton to come away with maximum points from the Stadium of Light.
It leaves Sunderland heading into the two-week international break with their plight looking increasingly desperate and Allardyce admitting he faces a "mammoth task" to steer the club to safety.
Allardyce said: "If a result goes against us, we've got to try not to let it drain our confidence too much.
"We've got to keep that fire in our belly and that determination to get out of the position we're in.
"In training, I see a lot of good stuff, a lot of good things, but I'm not seeing it when we get out on the field.
"There's got to be a bit of fear in the players; frightened to express themselves.
"They've got to get rid of that fear and show on the pitch what they can do on the training ground.
"We've got a lot of games to get out of that position, but we've got to do what we did against Newcastle, particularly against the teams who are going to be in or around us."
In a dire spectacle, Southampton dominated possession, yet Sunderland were improved defensively, albeit Costel Pantilimon needed to make a couple of smart saves, while Sebastian Coates and DeAndre Yedlin cleared off-the-line.
But a self-inflicted blow again proved fatal to Sunderland after Yann M'Vila conceded a penalty midway through the second half with an utterly needless tackle on Ryan Bertrand.
"To give away a soft penalty was the biggest sickener for us," added Allardyce.
"Obviously, we didn't play as well as I'd have liked, but we have to accept the team we were playing against is a very good one.
"But to give that penalty away after we'd defended so well, was a real blow for us.
"Southampton were better than us, there's no doubt about that, but then you only need one slip from them even if you don't deserve it.
"But we did the slip, not them.
"He (M'Vila) didn't need to go to ground, he's done the job and covered around DeAndre (Yedlin).
"You just need to jockey the player out the box.
"It seems to be a part of our make-up though; a silly mistake costing us dearly."