Howe gets some breathing space
Eddie Howe's declaration that this is the defining period of Bournemouth's season drew exactly the response the manager had hoped.
Goals in each half from Steve Cook and Benik Afobe ensured Howe's side ended a run of three successive home defeats to take a significant step towards securing Premier League survival. Victory over their neighbours means they will now face fellow strugglers Newcastle and Swansea strengthened by the belief they are now moving in the right direction.
For Southampton, though, a second successive defeat weakened their hopes of a Europa League place and the loss came at the extra cost of injuries to Charlie Austin and Shane Long, who both limped off in the second half.
The game was also held up briefly after fourth official Kevin Friend collapsed and banged his head against the dug-out. He was taken to hospital after regaining consciousness.
Matt Ritchie and Joshua King posed a constant threat and the opening goal came on 31 minutes when King exposed the weakness on the left of the Saints defence.
The forward proved too strong for Jose Fonte, before drawing a foul on the edge of the area. Ritchie drilled the ball in from a tight angle and Fraser Forster did well to save, only for Cook to volley in.
Koeman introduced Steven Davis and the Saints exerted more control after the break. But their hope vanished when Afobe headed in Ritchie's 79th-minute free-kick.