Frank Lampard's side were outstanding in Sunday's victory over Tottenham but fell well short of the same standards four days later and could have few complaints after falling a third defeat in four top-flight matches at Stamford Bridge.
Michael Obafemi, who started as Southampton rested the in-form Danny Ings for the first of two games in 48 hours, lit up a poor first period with a fine run and finish after 31 minutes.
OBAFEMI 👏 pic.twitter.com/PN7SLFlaUU— NBC Sports Soccer (@NBCSportsSoccer) December 26, 2019
Chelsea upped their tempo after the interval following the introduction of Mason Mount and a switch to a back four, but the Saints - who have now won back-to-back away games to climb away from the relegation zone - were worthy winners and grabbed a second goal when Nathan Redmond finished a wonderful move with 17 minutes remaining.
Neither goalkeeper was forced into a meaningful save in a dire opening half-hour as Chelsea dominated possession but offered a distinct lack of quality in the final third.
The hosts were then stunned after Callum Hudson-Odoi was dispossessed near the halfway line. Saints skipper Pierre-Emile Hojbjerg played an incisive ball into Obafemi and the striker quickly turned and drove forward, cutting inside Kurt Zouma, before curling a glorious left-footed finish into the top-left corner.
Alex McCarthy pawed away a Chelsea corner that had been flicked on at the near post eight minutes later and Tammy Abraham fired into the side-netting at the start of the second half.
However, it was Southampton who continued to produce the best attacking play despite their limited share of possession.
Fikayo Tomori got a key touch to prevent Che Adams connecting with another cross from Redmond, who also drew a save from Kepa Arrizabalaga.
With Chelsea labouring at the other end, a flowing Southampton move then sealed the points, Redmond dinking the ball over Kepa after great work from Stuart Armstrong.
REDMOND SILENCES STAMFORD BRIDGE 😳 pic.twitter.com/A5ghmmvjhK— NBC Sports Soccer (@NBCSportsSoccer) December 26, 2019
What does it mean?
Chelsea's first half a season under Lampard has had many highlights, but improving the club's home form will be a clear priority for 2020. As has been the case in several home games this term, this was nowhere near good enough.
Back-up forwards shine for Saints
With Ings rested and Shane Long (knee) injured, Obafemi and Adams led the line for Southampton and both performed impressively. Obafemi's goal was glorious, while Adams worked tirelessly and used his strength to good effect.
Wing-backs typify lack of Chelsea quality
Chelsea's starting wing-backs, Cesar Azpilicueta and Emerson, were repeatedly found in promising positions in the first half, but their end product was poor. It was no surprise to see Lampard abandon his back three at the break, albeit Chelsea remained poor thereafter.
Chelsea face an eye-catching trip to Arsenal on Sunday for Mikel Arteta's first home game in charge of the Gunners. Southampton are at home to Crystal Palace on Saturday.