It is fair to assume that Valentine's Day has lost some of its sparkle for the few hundred Chelsea supporters who travelled to Prague for this tie. It had promised little, apart from the novelty of seeing the club in Europa League action for the first time, and the reality in the sub-zero temperatures was even bleaker.

Thank heavens for Oscar, the 82nd- minute substitute, who scored a lovely goal within seconds of his introduction, and for the interim manager Rafael Benítez, who will surely be hailed for what was a game-defining change. Or maybe not.

The devoted souls in the away enclosure had vented their irritation at Benítez during the second-half as they endured a desperately tedious encounter that looked set to be remembered for the further toils of Fernando Torres. But Oscar brought belated cheer and the prospect of serene passage in next Thursday's return in London, which stands to fulfil the stag do double-header with a last 16 meeting against Ajax of Amsterdam.

The Brazilian ran on from the touchline and almost immediately, he had swapped passes with Eden Hazard and showed his trademark balance and technique to swerve past his marker and open up the chance. His finish was left-footed, sweet and out of keeping with almost everything that had gone before it.

There had been plenty of time to ask searching questions. What were the European champions doing here, in front of 18,952 fans, in the week when the Champions League last 16 had enthralled us? What was anybody doing here? Did anybody care? For long spells, it seemed as though few people did. But Oscar's intervention created a rosy glow. It was the first time this calendar year that Chelsea had won back-to-back fixtures, after Saturday's home win over Wigan Athletic. Perhaps, there is something to build on.

Chelsea had flickered at the outset, with Hazard and Marko Marin getting into dangerous areas and the near sell-out crowd, who had pitched up for Sparta's biggest game in years and their first since 6 December and the Czech league's winter hibernation, did not exactly create a pulsating atmosphere.

But Chelsea came to show alarming defensive vulnerability and Sparta were the better team in the first-half. But for David Lafata's loose finishing, the story could have been different. The winter break signing from Jablonec blew the chance to mark his debut with a goal on 16 minutes, when he fastened unattended on to Tomas Zapotocny's cross. He lifted his shot over the crossbar.

Chelsea lived on the finest of margins and there were nervy moments in their backline, with Petr Cech also catching the bug. Ladislav Krejci went down inside the area after going shoulder-to-shoulder with César Azpilicueta; Lafata was pulled back for offside and Krejci was adjudged to have run the ball out of touch before his cutback for Vaclav Kadlec. Lafata also dragged another excellent shooting chance wide.

Torres was on the end of Chelsea's brightest moments in the first-half, which was annoying, given that he was in one of those moods when he looked disinterested and nothing would go right for him. Twice, Hazard crossed dangerously but Torres could not read his intentions and when Juan Mata opted to pass to him, rather than shoot when well-placed on the break, Torres' weak effort never threatened. The striker stubbed another shot before half-time.

Benítez's defence of Torres was the personification of stoicism. "We have to create more opportunities and then he will score more goals," he said. "His work rate was very good and thanks to him, the offensive line could create. His job was crucial to run and keep the defenders deep to make that space for the other players. Everybody expects a striker to score goals but he is also doing a great job for the team."

It was easy to feel the bitter cold and not only because of the lack of spirit-warming entertainment. Still, Benítez braved the elements in nothing more than a suit and shirt; he has long since eschewed the flak jacket. The first murmurs of dissent from the Chelsea fans came in the 53rd minute, when they told Benítez, impolitely, to leave, as he was not wanted. It has been the soundtrack to his interim tenure. Right on cue, Marin crossed and Torres failed to connect with his volley. It was deeply frustrating.

There was the usual vociferous support of Frank Lampard and the stand-in captain headed a half-chance at the goalkeeper Tomas Vaclik from Mata's cross. It was a rare moment of second-half enterprise and when Torres shot so far wide in the 71st minute that the ball went out for a throw-in, the evening felt complete. But there was a sting, when Oscar made his dramatic entrance.

Chelsea were more solid at the back in the second-half and Benítez felt that they showed better "concentration and intensity." Torres even finished by drawing a smart save out of Vaclik. The full-time whistle, though, came as a relief.