Ronald Koeman said it was "impossible" for Chelsea to defend Sadio Mane, as his Southampton side saw off Jose Mourinho's team 3-1 at Stamford Bridge on Saturday.
Mane scored the second goal of the game, taking a pass from Graziano Pelle and turning inside of Chelsea's John Terry in the penalty area to finish past Asmir Begovic.
Captain Terry was restored to Chelsea's lineup, but Saints exposed the now familiar failings in the defence which just five months ago formed the foundation for winning the title.
Goals from Steven Davis, Mane and Pelle earned Southampton a first win at Stamford Bridge since January 2002.
"The whole team performance was perfect,'' Koeman said. "After 1-0 down we looked comfortable and of course to win against teams like Chelsea you need your best players on that high level.
"Sadio was dangerous in his movements. It was impossible to have good defending against him.''
Mourinho complained about Radamel Falcao being denied a penalty, even though Mane had a stronger claim. Koeman hit back at Mourinho's penalty comments, saying: "Maybe if they had one penalty we [should have] had two penalties -- that's a decision for the referee.
"In my opinion, we totally deserved three points today. That's a confidence everybody needs. If we can show this against Chelsea, we can show it much more."
Mourinho all but conceded defeat in the title race following the loss, with Manchester City 10 points ahead in the table.
"To be champions will be very difficult. Ten points [to] Man City, maybe tomorrow 11 points [to] Man United,'' Mourinho added.
"In football it's not impossible, but I think very difficult. Top four is completely -- I don't say easy -- but doable.''
On the dissent from the supporters, Mourinho said: "If they have some negative reaction, I would do the same, because the results are not the results that we want.''