David Moyes was adamant Manolo Gabbiadini's opening goal should have been chalked off - but he was refusing to use that as an excuse after watching his Sunderland side thrashed.
Sunderland were comfortably beaten 4-0 by Southampton, with Moyes left deeply frustrated by his side's lacklustre showing a week after they put four past Crystal Palace.
Defeat leaves Sunderland bottom of the Premier League, with the gap to safety still two points after defeats for Hull City and Palace.
Sunderland fell behind to Gabbiadini's 30th minute goal, the Italian putting the ball past Vito Mannone with his arm.
Sunderland then fell apart, Gabbiadini adding a second in added-on time before a Jason Denayer own goal and Shane Long strike adding to Sunderland's misery.
Moyes said: "I am really disappointed that is the way it worked out.
"Sometimes when the expectation has gone on us to win, or maybe people think [we should], we fluffed our lines a bit and it happened that way today.
"I am really disappointed with the result and the performance, especially in the second half.
"We had played well for 20 to 25 minutes, maybe as well as we had here this season, at times, without making enough opportunities.
"The goal goes in, the first is really important, Southampton are a team that had been struggling for wins. The first was always going to be really important for both teams.
"The boy puts it in with his arm, it doesn't touch his head or anything like that, it goes straight in off his arm.
"It was a great cross in by Ryan Bertrand and it is difficult for the referee to see it but goes in off his arm - it shouldn't have been given. That changes the way it goes.
"The second goal we didn't get tight enough in the box and it puts us in a situation where we have to make changes at half time and try and change it.
"We couldn't stick with it and hang in, we had to try something different. The more attacking players we put on, the worse we became. That is for sure."
Sunderland are not in action again until they travel to play Everton two weeks today.
When asked whether Sunderland were now back at square one after the heavy defeat to Saints, Moyes said: "At least we are in a square.
"We are in with three or four other teams, it won't be easy but I was worried a bit about today.
"I know Southampton are one of the better footballing teams in the league and if you give them a chance, once they got a couple of goals up they could wait and pick us off.
"The more we tried to get a goal back, or do something else, there was more chance we were going to lose more goals."