The game seemed to be going very much to plan for the Saints, as they took the lead through Shane Long just before half-time, shortly after goalkeeper Fraser Forster had achieved the record of the longest run without conceding a goal for the club.
Chelsea punish Saints' lack of control
Chelsea, who have vastly improved from the side that were dismantled by the Saints in the reverse fixture back in October, were not going to let the home side run away with the match, and quickly began to assert themselves on the game from the beginning of the second half.
A 75th minute equaliser from Cesc Fábregas was then followed by a late winner for the blues as defender Branislav Ivanovic's header beat Forster, with just one minute of normal time remaining.
On the Saints first defeat since January 2nd Koeman said: "We did well, but it was not good enough to the end and they punished us."
The Dutchman continued: "We lost a bit of control in the second half and Chelsea had more possession."
The Saints boss also said that lack of possession and control in the second half meant they were unable to build on their slender lead: "To get the second goal you have to create chances and we lost the ball more in the second-half."
Result into context
The defeat will bring Saints' fans, who were dreaming at the possibility of snatching fourth place, back down to earth as the defeat now leaves the south coast side in seventh place.
Koeman's men are seven points behind Manchester City, who occupy fourth spot, although the Manchester club now have a game in hand as they take place in the Capital One Cup Final this weekend, meaning the Saints could find themselves ten points behind the well coveted Champions League place.
Southampton will have to pick themselves up quickly as they turn their attentions to the small matter of a south coast derby with Bournemouth on Tuesday night.