The Blues will have to overturn a 1-0 deficit at Stamford Bridge next week if we are to progress into the Europa League quarter-finals after a Raul Rusescu penalty proved the difference between the two sides at the National Arena.
Steaua dominated the first half in terms of possession, and they were rewarded in the 33rd minute when Ryan Bertrand conceded a penalty for a foul on Rusescu as they challenged for an aerial ball.
Rusescu dispatched the kick to put the hosts in front, and while we improved after the break, and went close on a couple of occasions through Eden Hazard and Marko Marin, we were unable to level the game and score what would have been a crucial away goal.
It will take a big performance in front of our own supporters to swing the tie back in our favour, but having turned around bigger deficits in the past, the players will feel confident of overcoming the Romanian side in west London.
Immediate attention now turns towards Sunday's FA Cup sixth round tie away at Manchester United.
Rafael Benitez made five changes from the team which beat West Bromwich Albion at the weekend. Into the back four came captain John Terry and Bertrand, while Cesar Azpilicueta and Ashley Cole had to be content with places on the bench. In midfield, John Mikel Obi replaced Ramires alongside Frank Lampard, who was making his 593rd appearance for the club, becoming our third highest appearance maker of all-time. There were also starts for both Yossi Benayoun and Fernando Torres.
Backed by a fervent home support, Steaua were quick out of the blocks, and after Benayoun had hooked away a first-minute corner, Alexandru Chipciu fired wide from distance as they looked to gain an early advantage in the tie.
The Blues were struggling to get on the ball early on as the hosts sprayed it around confidently, with Alexandru Bourceanu pulling the strings in midfield and Adrian Popa threatening down the right-hand side.
The individual battle between Popa and Ryan Bertrand, who had started the game well, had all the makings of a pivotal contest.
Ryan Bertrand in action against Steaua
Our only sight of goal during the opening 20 minutes saw Torres get away from Iasmin Latovlevicion on the right-hand corner of the penalty box, but as the Spaniard shaped to cross he miss-controlled the ball and saw it run out of play.
For all Steaua's possession, however, Petr Cech was yet to be tested as the half-hour mark approached, with the Chelsea defence, marshalled by Terry, dealing with the steady flow of crosses from wide areas relatively comfortably.
As the half progressed, Torres was beginning to ask questions of the left-back Latovlevici, and soon after the defender was booked for one particular challenge, he was then forced into a last-ditch tackle as the Chelsea striker attempted to get beyond him and have a clear run at goal.
Twelve minutes before the break, though, the home side took the lead.
Latovlevici delivered a cross to the far-post with the outside of his boot, and as Rusescu climbed to meet it, Bertrand was adjudged to have pulled him back, handing Steaua a penalty and earning the Blues left-back a yellow card.
At first glance, it looked the correct decision, but replays showed that the Chelsea youngster was unfortunate to see the call go against him.
Rusescu was the man entrusted with taking the resulting kick, and after dusting himself down he made no mistake, firing powerfully past Cech into the bottom corner.
Terry, Mikel and Torres
If any positives were to be taken from falling behind, they came in the manner of our response, and five minutes later we were almost level.
Hazard, who had been a peripheral figure for much of the first half, did well to win possession, and as he fed Benayoun just inside the box, the Israeli's shot, which looked destined for the back of the net, was somehow clawed away by Ciprian Tatarusanu.
Four minutes before the break Steaua once again had their goalkeeper to thank for maintaining their lead as he palmed away a Torres effort after the striker had taken it well on the bounce.
In truth, it had been a disappointing display during the opening 45 minutes, and there was plenty for Benitez to mull over at the break.
Both managers opted against making any changes at half-time, and it was Laurentiu Reghecampf's side who almost doubled their lead shortly after the restart when Cristian Tanase volleyed narrowly wide.
While we were pushing forward in a bid to get back into the game, attacks were continually breaking down in the final third, largely down to a combination of resolute defending from the home side and a poor final ball from ourselves.
The creative trio tucked in behind Torres were finding it difficult to influence proceedings, and on the one occasion the striker did breach the Steaua defence, latching on to a clever Lampard ball, he opted to pass when he should have taken the shot on.
Sensing the need for an injection of fresh legs, Benitez made his first substitution of the night just after the hour, replacing Benayoun with Juan Mata, and it had a positive effect.
Eden Hazard in action against Steaua
With the clock ticking down we were beginning to turn the screw, and 17 minutes from time our best opportunity of the half went begging.
Lampard and Bertrand exchanged passes wide on the left, and as the midfielder sent a hopeful cross into the box, Torres did well to head it into the path of Hazard who fired inches over the bar.
It was to be the Belgian's last involvement as he was replaced minutes later by Marko Marin, and the substitute almost had an instant impact, forcing a decent save from Tatarusanu at his near-post.
Next to go close was David Luiz, who saw his long-range free-kick drop just over the bar, and it was clear from the reaction of the home supporters every time we were in possession that they sensed the game was far from over.
Four minutes of added time were signalled, but we were unable to profit from our possession, and in the dying minutes it was Steaua who looked the side most likely to score again, leaving us with much to do in next week's return leg.
Chelsea (4-2-3-1): Cech; Ivanovic, Terry (c), David Luiz, Bertrand; Mikel, Lampard; Benayoun (Mata 64), Oscar, Hazard (Marin 74); Torres.
Unused substitutes: Turnbull, Ferreira, Azpilicueta, Cole, Cahill.
Booked Bertrand 33, Mikel 66
Steaua Bucharest (4-2-3-1): Tatarusanu; Rapa, Szukala, Chiriches, Latovlevici; Pintilii (Prepelita 57), Bourceanu (c); Popa, Chipciu, Tanase (Tatu 81); Rusescu (Gardos 90+1).
Unused substitutes: Stanca, Filip, Iancu, Sobrinho.
Scorer Rusescu 34 pen
Booked Pintilii 49
Referee Sergei Karasev (Russia)