Mark Hughes was frustrated with Stoke City's defending from set-pieces after they lost 2-1 at home against 10-man Southampton to lose ground in the race for continental qualification.
Graziano Pelle headed Southampton into an early lead from a corner and the striker curled in a second goal before half-time after playing a one-two with Dusan Tadic.
Marko Arnautovic scored shortly after the interval to give Stoke hope but they could not force an equaliser despite Sadio Mane's late red card for a mistimed aerial challenge on Erik Pieters.
"We lost the game in the first half - there is no doubt about that," Hughes told the club's official website.
"We spoke about their prowess from corners and set-pieces earlier in the week, because they have scored any number of goals this season similar to the one they scored today.
"It was highlighted to the guys that they needed to do their jobs and stop their players from making those runs inside the box, but individually and collectively we failed to do that.
"We conceded from the very first corner, which is something that should never happen and at that point we are 1-0 down, which is a difficult position to be in.
"I have spoken until I am blue in the face about the importance of scoring the first goal, but when you concede a second then it is really an uphill task for us."
Hughes was happier with his team's reaction after half-time but Stoke's defeat means they slip to ninth in the Premier League after their four-game unbeaten run came to an end.
"I thought we were a lot better in the second half, but after what had happened in the first half we gave ourselves too much to do," the Welshman added.
"Unfortunately that has happened on too many occasions this season, especially at home, so we have got to work at that and make sure we cut it out moving forward."