Nuno Espírito Santo admitted West Ham United's painfully slow start cost them after they crashed to a 3-2 home defeat by Southampton.
The Hammers, preseason promotion favourites, were a goal down after 100 seconds and two behind after 10 minutes thanks to strikes from Kuryu Matsuki and James Bree.
Kyle Walker-Peters pulled one back against his former club but Lewis Dobbin restored Southampton's two-goal lead before half-time.
Substitute Tomás Soucek scored another in stoppage time but West Ham had given themselves too much to do as their nine-match unbeaten run ground to a halt.
"We started not the way we wanted and we conceded," Hammers boss Nuno said.
"We were looking at the situations with hesitation, bouncing balls, too many chances for Southampton to have a second phase, third phase and achieve a goal.
"We were not good in that moment, but the reaction was good. After you start playing, you start combining, you create enormous situations.
"The score was still on top of us, though, and the third goal punished us. Overall, in the first half, we didn't do things well enough to try and dominate and control Southampton.
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"In the second half, the response was good. We tried, many situations that we could have done better. The boys fought until the end.
"The fans helped us. All of us were having the feeling that we were able to react and revert the game, but it was not to be. It was disappointing."
West Ham could have gone top with a win but they now find Southampton, who were deducted four points at the start of the season, just two points behind them.
Saints boss Tonda Eckert said: "This is a tough place to come, against a quality team, and we knew we needed to start strongly, so we worked on that all week.
"That costs a lot of energy, so we had to make sure we went up in that period.
"They made it tough for us in the second half, so it was important that we stayed brave to bring the result over the line.
"We were clinical today, more than in our last three games, and you could see the togetherness of the whole group."
