Last Updated: Mon, 02 Dec 2013 08:14
Mauricio Pochettino was disappointed that Southampton conceded two set-piece goals during their 3-1 defeat at Chelsea on Sunday.
After Jay Rodriguez's opener inside just 13 seconds, Gary Cahill, John Terry and Demba Ba scored in the second half as Jose Mourinho's unbeaten Premier League home record was extended to 67 games.
"It's clear that the game changed when we scored that goal so early on," Pochettino said.
"I think we competed well and even up to them getting even, I think that was our best spell during the game because we were attacking.
"Then we conceded two goals from set-pieces. That cannot happen.
"That's a mistake that shouldn't happen in football. They were soft goals we shouldn't have conceded."
Artur Boruc had done well to deny Chelsea, but the goalkeeper departed after Cahill's equaliser and went to hospital for X-rays on his hand and wrist.
After a first defeat in nine games at Arsenal, Saints have now lost to two of the title contenders who Pochettino says cannot be split.
"They're clearly two teams that are going to be in the fighting to win the league," Pochettino said.
"Both teams have their strengths, have their weapons to inflict a lot of danger to their opposition."
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