Oriol Romeu admitted the difference between Chelsea and Southampton was that the Blues were far more clinical.
Antonio Conte's in-form side triumphed 2-0 at St Mary's, inflicting a first home defeat of the season on Saints, thanks to goals in either half from Eden Hazard and Diego Costa.
Romeu said: "It was a tough, tough game. They are very clever with what they do. Once they scored they defended in their own half and were then coming out.
"We tried to stop them, but after a simple mistake again it's another goal and against this team they are very clinical up front. We needed to play perfectly defensively and today I think we made those two mistakes that gave them the chances to go ahead."
Romeu admitted conceding in the sixth minute to Hazard made an already-difficult challenge all the harder.
"It's tough to start losing that early in the match, but I think we reacted quite well," he said. "Our first half was very good, with a good attitude and good football.
"We couldn't have many chances, and I don't think they had too many either but it was a clever game from them and they were more clinical up front than us."