Southampton's 3-0 home loss to Everton boiled down to a lack of organisation, according to manager Ronald Koeman.
The hosts succumbed to Romelu Lukaku's first-half double and an excellent late strike from Ross Barkley, with Everton exploiting Southampton's vulnerability to the counter-attack.
"Maybe we had one or two good chances, but the performance was not what we expected, not defensively," Koeman, who was back in the dugout after ankle surgery kept him out of the season-opening draw at Newcastle United, told BT Sport.
"We weren't aggressive enough in our defending. We gave too much space to Everton to play. You need better organisation, at least you need better reaction of the players.
"We give a lot of attention to set-pieces, offensively and defensively. You need that organisation and we didn't have a good kind of organisation – you can't do that at this level."
Southampton's next test sees them take on FC Midtjylland at St Mary's Stadium in Thursday's UEFA Europa League qualifier.
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