Southampton conceded twice away from home for the first time since the opening day of the Premier League season as they lost 2-1 to Stoke City at the Britannia Stadium.
Morgan Schneiderlin scored his 15th goal for the club to open the scoring, but Mame Diouf’s goal just after the restart brought Stoke back into the game.
Charlie Adam, who had come on at half-time, then found space in the box to hammer home the winner as Saints suffered their first ever Premier League defeat to the Potters.
Koeman made two changes from last weekend’s 2-0 win against Hull City, as Nathaniel Clyne and Dušan Tadić returned to the side in place of Shane Long and the suspended Victor Wanyama.
With Wanyama missing it was Toby Alderweireld who moved into midfield alongside Schneiderlin, with Sadio Mané deployed centrally behind Graziano Pellè.
It was Mané who was involved in the game’s first real incident. The Senegalese international was found by Pellè in the box and barged over by Stephen Ireland, but referee Mark Clattenburg was unmoved despite strong appeals from the visitors.
Mané then played in Pellè with a clever ball behind the defence but, despite Saints fans at the other end thinking he’d opened the scoring, his strike only found the side netting.
The visitors had been threatening and took the lead midway through the first half. José Fonte flicked on a corner from Steven Davis and Schneiderlin was on hand at the back post to tap home for his fifth goal of the season.
Saints, through Mané in particular, were looking lively on the counter and twice the 23 year-old did well to create opportunities for his teammates.
The first saw Pellè find space in the box to turn, but he could only fire straight at Asmir Begovic before Nathaniel Clyne blazed over five minutes later.
Stoke’s first clear-cut opportunity came just before half-time. Marko Arnautovic’s free-kick was flicked on to Steven N’Zonzi, but he was unable to connect cleanly with the ball and it trickled wide.
It was N’Zonzi however who played a key role in Stoke equalising just two minutes into the second half. His innocuous shot came back off the crossbar into the path of Diouf, who smashed home.
After a spell of dominance from the home side following their equaliser, Saints should have gone back ahead just after the hour mark.
Mané was again instrumental, drawing Begovic out of his goal before teeing up Tadić, but Glenn Whelan cleared the Serbian’s effort off the line.
As Saints pressed for a winner, they were undone at the other end with substitute Adam taking full advantage of confusion in the Saints defence.
After Jon Walters had seen his shot blocked it fell kindly for Adam, who fired the ball beyond Kelvin Davis.
Koeman threw Shane Long and Eljero Elia on in search of a late equaliser, but it wasn’t to be for Saints, who suffered further disappointment with Alderweireld needing to be stretchered off in stoppage time with a shoulder injury.
Stoke City: Asmir Begovic, Geoff Cameron, Ryan Shawcross (c), Philipp Wollscheid, Erik Pieters, Steven Nzonzi, Glenn Whelan (Steve Sidwell 76), Jonathan Walters, Stephen Ireland (Charlie Adam 46), Marko Arnautovic, Mame Diouf (Peter Crouch 76). Substitutes: Jack Butland (GK), Phil Bardsley, Marc Wilson, Dionatan Teixeira.
Goals: Diouf (47), Adam (84)
Southampton: Kelvin Davis, Nathaniel Clyne, José Fonte (c), Maya Yoshida, Ryan Bertrand (Shane Long 86), Toby Alderweireld, Morgan Schneiderlin, Steven Davis (Eljero Elia 86), Sadio Mané, Dušan Tadić (James Ward-Prowse 67), Graziano Pellè. Substitutes: Paulo Gazzaniga (GK), Florin Gardos, Matt Targett, Harrison Reed.
Goals: Schneiderlin (22)
Booked: Fonte (15), Clyne (38), Alderweireld (50), Elia (88)
Attendance: 26,467 (1,830)