The Saints went down 2-1 to West Ham at the Boleyn Ground in the Premier League.
An own goal from Arsenal-owned defender Carl Jenkinson saw the Saints take the lead after just 13 minutes, but second-half goals from Michail Antonio and Andy Carroll helped the Hammers register their first league victory in two months.
Over the course of the 90 minutes at the Boleyn Ground, hosts West Ham had 49% of the possession, took 13 shots of which just five were on target, and earned four corners.
Visitors Southampton had 51% of the possession, took 15 shots of which only four were on target, and earned seven corners.
Koeman has taken to Twitter to give his reaction to the loss, with Saints fans responding to his message.
Below are some of the best comments:
Disappointed final result! If you play so well during 45 min you have to score more than 1 goal! More mistakes in 2 half! #WeMarchOn— Ronald Koeman (@RonaldKoeman) December 28, 2015
@ronaldkoeman Trust in Seager (& Gallagher when back) There's no point developing these lads if we don't play them. Well done for 2015, tho.— Simon De La Rue (@sjdelarue) December 28, 2015
@RonaldKoeman then please sign an established premier league striker to push on in the new year.— Bannerman (@saintbannerman) December 29, 2015
@RonaldKoeman so do something about it then? We can all analyse a game, you make such ridiculous substitutions.— #getyoshidaoutasoton (@r0blacey) December 29, 2015
@RonaldKoeman Finishing has to improve. If everything else stayed the same but finishing improved we'd still get win more games.— Martin St. Hill (@MartinStHill) December 28, 2015