Ronald Koeman lamented Southampton's failure to deal with the impact of West Ham substitute Andy Carroll as the Dutchman oversaw a 2-1 Premier League defeat at Upton Park.
The visitors took the lead when Carl Jenkinson turned Shane Long's cross into his own net under pressure from Dusan Tadic in the 13th minute, but West Ham fought back as Michail Antonio fortuitously levelled and Carroll headed the winner 11 minutes from time.
Having been much the better side in the first half, Southampton's challenge faded after the interval and Koeman – whose side had been so impressive in the 4-0 Boxing Day win over Arsenal – conceded that Carroll's introduction changed the game.
"Football is not 45 minutes, football is 95 minutes and the second half was totally different," the Dutchman told BBC Sport.
"The first 20-25 minutes of the second half we lost control. When Carroll came in he changed the confidence of the West Ham team, because the first 45 minutes Southampton were by far the dominant team.
"We spoke at half-time about how they will give a reaction, they will bring in Carroll, which means they will play the long ball, they will fight for second balls, they will be more aggressive, and then you need to concentrate.
"If you play so well for 45 minutes and you get some very good chances you have to score more than one and we didn't."
Southampton's triumph over Arsenal is their only victory in the last seven league games and they sit 12th.
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