Any fanciful hopes that José Mourinho would wave his magic wand and right all that was wrong in these parts have now been dispelled. A little over a month since he returned triumphantly the scale of the task he has taken on is clear: the final whistle prompted boos from the home support as the players from Basel celebrated a first win on English soil. Mourinho, head bowed, departed down the tunnel with defeat hard to digest.

Not since Besiktas back in October 2003 had Chelsea, under Claudio Ranieri, succumbed at home in the group stage of the Champions League. This was more than a jolt to the system. It was a migraine-inducing reminder of this team's fallibility, a position of relative authority having been surrendered wastefully; even attempts to salvage a point were rather unconvincing and laced with panic. The last time Mourinho had overseen Chelsea in this area in European competition, against Rosenborg six years ago, they had been jeered off and he was sacked within 48 hours. In that context Roman Abramovich's march across the pitch and into the home dressing room post-match seemed ominous.

At least there is an apparent acceptance within the hierarchy these days that this campaign will have moments like this, with Mourinho having been at pains to stress his is a young team in transition and a side, as he suggested again here, that "lacks maturity" to recover from setbacks. Whether that is fair, given Chelsea have claimed a European Cup and a Europa League in the last two seasons and that the average age of their starting line-up was almost 28, is open to question.

Yet the mental fragility was clear. Their performance through much of the opening period had been stodgy at best and Oscar's goal on the stroke of half-time was plucked just as the Swiss relaxed for the first time. But there was carelessness in an inability to capitalise on that, and the first coherent attacking move mustered by the visitors cut Chelsea to shreds.

The equaliser was slickly constructed, the ball shifted smartly from left to right at pace with home defenders lunging in but unable to intercept, before Mohamed Salah curled a delicious shot beyond Petr Cech. The winger had been pesky before that, tormenting Ashley Cole whenever offered an opportunity to charge into space beyond the full-back, and had scored in last season's Europa League semi-final here between these sides. That reward had been overtaken by a glut of subsequent home goals. Here it merely pepped Basel's resolve.

Eight minutes from time Salah broke again and Marco Streller's near-post attempt was deflected behind for a corner. Kay Voser's near-post delivery was flicked in by Streller, the striker having benefited from a tangle between Gary Cahill and Samuel Eto'o in the six-yard box that served to liberate him from his marker, and the Swiss had their win.

Their manager, Murat Yakin,, attempted to maintain his deadpan calm post-match but could not help but enthuse at his team's performance, and particularly their resilience and recovery after the interval while his players celebrated raucously with the stereo blaring in the nearby dressing room. His introduction of the much criticised Matías Delgado had added bite to their pursuit and his game-plan frustrated Chelsea throughout. Even Voser and Behrang Safari deserved credit for recovering some poise having been embarrassed in the early exchanges by the galloping Willian and Eden Hazard. The wingers promised much but delivered little while Eto'o is still rusty, not sharp enough yet to capitalise on the movement his brain is so willing him to instigate. Juan Mata was introduced at 1-0 and offered little while Fernando Torres was not even in the match-day squad. Oscar alone of the home players posed a persistent threat and, while he purred, Chelsea had threatened to prosper.

The Brazilian, one of the three players 22 or under whom Mourinho has selected – he had promised four "beautiful, young eggs" in the build-up – had conjured the lead from nothing while the Swiss contemplated some half-time satisfaction. David Luiz was permitted to amble forward, Frank Lampard collected possession and slipped a pass inside Safari and Oscar's first-time finish back across Yann Sommer and into the far corner was supremely accurate.

The playmaker enjoys this stage and had scored twice against Juventus in the corresponding fixture last season, only to see that lead hauled back to 2-2. It turned out to be worse this time, though he was unfortunate before the hour as he collected possession and arced a shot on to the crossbar from just outside the corner of the penalty area.

Memories of that flash of brilliance were dulled in defeat, a fourth match without a win exposing the work that must be done to re-establish this squad as contenders on all fronts. Mourinho rightly said that this result does not make progress into the group stage impossible but his team must surely claim all six remaining home points available and, potentially, also win in Basel in their penultimate game. Rafael Benítez's side did that in last season's Europa League. The boos that pursued the home players down the tunnel here were reminiscent of the general mood under the Spaniard; things were not supposed to be as grim with Mourinho back in charge but reality is biting.