Chelsea's campaign is supposed to have entered a defining period, a frantic phase that stretches to less than three weeks but is crammed with fixtures that will determine just what can be salvaged from a confused mess of a season. And yet, Rafael Benítez's insistence that he is "still confident we can finish in the top four and win one or two trophies" felt ludicrously optimistic. The sequence has begun with a splutter.

The European champions were outplayed for long periods on the south coast before succumbing to Southampton, a team with other priorities but with established form against those used to the thinner air at the top of this division. While the locals revelled in a victory that thrust them into mid-table, albeit still four points from the cut-off, those in the away section bellowed their disgust. The usual cries for José Mourinho went up, all merged with the chorus for Benítez to depart at pace. The interim manager is used to all that by now but, after a recent ceasefire, this was an ugly reminder that it only takes one debatable team selection, a questionable substitution and, certainly, a fitful performance to turn the mood sour.

This was a potentially damaging loss as Tottenham Hotspur and Arsenal prospered elsewhere, the gap to fifth place trimmed disconcertingly to two points ahead of the clutter to come. Chelsea could be out of the Champions League qualification places by the next time they trot out in the top flight, against Sunderland next Sunday, though they will have entertained Manchester United and Rubin Kazan by then. Victory here might have provided momentum in that run of daunting games, but all they have to carry into the FA Cup on Monday is a sense of deflation. There had been seven changes to the lineup from just before the international window but, where Benítez hoped for evidence of energy and inspiration, this was all too disjointed. It was even feeble at times.

Some of the cavalry were flung on towards the end, Ramires and Eden Hazard injecting some much needed zest, but they were chasing the contest by then and Southampton were in no mood to wilt. Kelvin Davis, called upon as the teams re-emerged after the interval when Artur Boruc complained of dizziness, tipped Victor Moses's scooped attempt over the bar and Frank Lampard's free-kick veered just over but, even in the latter stages, the hosts still created the more presentable opportunities.

"Rafa's one of the best managers in the world, and he has my respect for what he's done in football," Mauricio Pochettino, the Southampton manager, said. "I have sympathy for Rafa and recognise the job he's done." Few among the travelling support would agree, even if this logjam of games would test all-comers. Chelsea have secured a solitary point from four away games since mid-January.

How they missed Juan Mata here, the Spaniard out with a fever but hopeful of featuring against Manchester United. Gary Cahill may be absent, his knee still recovering from the damage sustained against West Ham earlier this month. John Terry, whose thumping downward header had briefly restored parity just after the half-hour mark, will expect to drop out again but this entire back-line laboured to contain Jay Rodriguez and Rickie Lambert.

The Premier League champions elect will presumably confront a team that starts with Ashley Cole and Hazard down the left, rather than Ryan Bertrand and Marko Marin, but they will not be too perturbed by what awaits in south west London.

Southampton will care little that they dismissed virtually a second XI. The displays mustered by Nathaniel Clyne and Jack Cork – who played 155 league games for six different teams while on loan from Chelsea, but not one for his parent club – summed up their wide-eyed enthusiasm and underlying quality, with the hosts superb throughout. They had prised their opponents apart far too easily when on the front foot, their dominance reflected midway through the first period as Lambert fed the ball in-field from the left flank for Rodriguez to collect. He exchanged passes with Steven Davis, a slick rat-a-tat to flummox static defenders, with the finish crisp and low beyond Petr Cech.

Even once pierced themselves, they recovered almost immediately. Chelsea were still celebrating Terry's riposte when Branislav Ivanovic fouled Lambert 25 yards out, and the forward's 14th goal of a productive first campaign at this level was curled in from distance; a fine way to savour a recently signed two-year contract extension. "He's a player I knew even before I arrived, and we're really proud of his performance," Pochettino said.

This club is starting to feel the same way for their man in charge. Chelsea have followed Manchester City and Liverpool in being dismissed since he took up the reins, and safety feels in sight. They can spend the week focusing on their reunion with Nigel Adkins next Saturday. Benítez, with fixtures flying at him from all angles, does not have that luxury.