Oscar scored within a minute of his introduction as Chelsea won 1-0 at Sparta Prague in their Europa League last-32 first leg at the Generali Arena.
An otherwise limp performance from both sides looked to be petering out into a draw until the Brazilian struck with eight minutes to play.
Despite a forgettable display, Rafa Benitez's men are now firmly in pole position to reach the last 16 when they welcome their opponents back to Stamford Bridge in a week's time.
Chelsea, who dropped into the Europa League after being dumped out of the Champions League as holders in the group stage, fielded a strong side with Frank Lampard, Juan Mata and Eden Hazard all featuring, though misfiring forward Fernando Torres had another game to forget.
"Sparta played well which made it good. It was a strange experience and I'm glad we got that one goal at the end," said goalkeeper Petr Cech, a former Sparta player.
A promising start from Chelsea, with Marko Marin lively, subsided quickly before a lapse in defence - all too familiar for Benitez of late - nearly presented Sparta with the opening goal after 17 minutes.
Right-back Tomas Zapotocny's left-footed diagonal pass over a static defence found David Lafata, who lifted the ball over Cech's goal when he had time to control the ball.
Moments later Ladislav Krejci shot and Cech had to dive to his right to save.
Lampard shot straight at goalkeeper Tomas Vaclik before the break while Torres, who now has one goal in 13 games, missed a straightforward effort.
After the resumption Lampard had to make a last-ditch interception to deny Krejci a shooting opportunity then Torres's evening was summed up when he slice high and wide - the ball going out for a throw-in.
With a stalemate looming and the visiting fans' ire for Benitez and Torres increasing, Oscar came on for Mata and scored with his first involvement, netting neatly following a one-two with Hazard.
Torres had an effort saved soon after before Ondrej Svejdik headed over from a corner at the other end as Chelsea held on.
MAN OF THE MATCH
Oscar - Chelsea: May only have been on for eight minutes, but did more than anyone else in a dour match to determine the result on the night.
SPARTA PRAGUE: Vaclik 7, Svejdik 6, Matejovksy 7, Lafata 6, Kadlec 7, Hybs 6, Zapotocny 6, Husbauer 6, Krejci 7, Holek 6, Vacha 6. Subs: Pamic N/A, Kweuke N/A, Bednar N/A.
CHELSEA: Cech 7, Azpilicueta 7, Ivanovic 7, Cahill 6, Bertrand 6; Ramires 6, Lampard 6; Marin 6, Mata 6, Hazard 6; Torres 5. Subs: Oscar 8, Benayoun 6.