Milton Keynes Under 16s followed a drubbing by West Ham United with an even more emphatic defeat at a Southampton team which swept aside all before them last season.
Said manager Paul Mortimer: "They are a good side, but we knew that going into the game, and as good as they are, they weren't five goals better than us.
"In the last two matches we just haven't performed though and we have to see how they respond.
"But we've only played two games and I'm not worried yet. I'd be worried if this was still happening after Christmas."
Southampton opened their account after three minutes and then added three more goals in a chaotic five-minute spell before the interval.
Jake Nash pulled one back after the restart when Tom Bounds' free-kick was spilled, but it was no more than a consolation.
Alongside Nash, midfielder Ted Day and strikers Sam Baldock and Ryan Beswick had decent matches.
The under 16s host Norwich on Saturday.