League group stage, are in danger of a second European exit of the season after losing in front of a full house at Bucharest's National Stadium.
Steaua, European champions in 1986, scored the only goal when Ryan Bertrand was penalised for impeding Rusescu at the far post and the 24-year-old forward stepped up to fire the penalty in the 34th minute.
Also on Thursday, Tottenham Hotspur trounced a sorry Inter Milan 3-0. Tottenham and Inter were meeting for the first time since the 2010-11 group stage when Inter raced into a 4-0 lead at the San Siro, before a Gareth Bale hat-trick in second half brought it back to 4-3.
Spurs then beat Inter 3-1 in return game.
Inter, fourth in Serie A, have been spectacularly inconsistent this season and suffered one of their off-nights, shoddy in defence and toothless up front.
Tottenham went ahead after only six minutes when man-of-the-moment Bale scored a rare headed goal, taking his tally to 10 for Spurs in his last eight matches.
Icelandic midfielder Gylfi Sigurdsson added the second from close range in the 18th minute after Jermaine Defoe's shot on the turn was parried by keeper Samir Handanovic.
Jan Vertonghen headed a third from a poorly-defended corner early in the second half.
Big-spending Russian pair Zenit St Petersburg and Anzhi Makhachkala failed to manage a goal between them.
Barca host Deportivo with an eye on Milan
Madrid: Barcelona assistant coach Jordi Roura has admitted that Lionel Messi could be rested when they host Deportivo la Coruna this weekend with their Champions League tie with AC Milan on Tuesday in mind.
The Catalans trail Milan 2-0 after the first-leg of their last-16 tie, but despite having such a big game on the horizon and an 11-point lead at the top of the table, Roura insisted it is vital for Barca to get back to winning ways against Depor.
"There is a long way to go in the league and although we have a good lead, we need to keep going game-by-game trying to win," he told reporters on Friday.
"How much time we have to rest between the games influences the line-ups a lot.
City take on Barnsley in FA cup
Manchester: Manchester City defender Pablo Zabaleta admits the Premier League champions have a lot to learn from this season as they look to overcome Barnsley in their FA Cup quarterfinal at Eastlands on Saturday. "As for this season, it goes without saying that we wanted to have a closer gap than 12 points," he said. "We have the FA Cup to go for as well, but obviously, when you look at the team you expect us to do better. So we need to learn from our mistakes but we are still learning and need to just carry on. Good teams need to keep winning," he said.