Juventus drew 1-1 at Manchester City and Italian teams, who failed to win in the opening round of games, produced a much better performance with Sampdoria and Palermo also claiming 1-0 home victories.
Moldova's Sheriff Tiraspol provided the night's biggest upset with a 2-0 win over Ukraine's Dynamo Kiev, helped by a bizarre opening goal, and Sporting Lisbon got the biggest win with a 5-0 demolition of Levski Sofia.
Napoli made a disastrous start in Romania, falling 3-0 behind in only 16 minutes as Cristian Tanase, with a free kick, and Pantelis Kapetanos added to a second-minute own goal by Emilson Cribani, who deflected a shot into the net.
But former European champions Steaua, in their first match under new coach Marius Lacatus, collapsed after Kapetanos was sent off in the 31st minute following a clash with Fabiano Santacroce.
Luigi Vitali pulled one back just before the break and Steaua twice hit the woodwork before substitute Marek Hamsik reduced the arrears in the 73rd.
Napoli's Uruguay forward Edinson Cavani completed the fightback by snatching the equaliser in the eighth minute of stoppage time to give the Italian side their second draw in two Group K games.
Liverpool lead the group with four points after an uninspiring draw at Utrecht who have been involved in two goalless stalemates after their opening draw at Napoli.
Atletico, beaten by Aris Salonika in their first Group B game, fell behind to a goal from Swiss striker Eren Derdiyok six minutes before halftime at home to erratic Germans Bayer Leverkusen.
Simao Sabrosa converted a penalty after the break to give Atletico their first point of the group, but the Spaniards are still bottom while Bayer lead with four points.
Juventus, amazingly held 3-3 at home by Poland's Lech Poznan, took an early lead at Manchester City when Vincenzo Iaquinta cut inside from the left and scored with a shot which clipped Kolo Toure's head and flew past Joe Hart.
Adam Johnson levelled before halftime in the Group A match when he slotted home Yaya Toure's pass. Juve nearly snatched the points when Alessandro del Piero's free kick hit the underside of the crossbar five minutes from time.
Lech Poznan went top of the group, ahead of Manchester City on goals scored, with a 2-0 win over hapless Austrian champions Salzburg, thanks to second-half goals from Manuel Arboleda and Slawomir Peszko. Poznan and City each have four points.
Colombian forward Radamel Falcao scored Porto's only goal of the night, flicking the ball in from Hulk's pass to give the Portuguese side three points and keep them top of Group L.
Sampdoria's Giampaolo Pazzini scored from the spot as they beat visitors Debrecen 1-0 in Group I to claim the first win for an Italian team in this season's competition, while PSV Eindhoven won 2-0 at Metalist Kharkiv in the same group.
Italy's Palermo also won but it was far from convincing as they needed a goal from Giulio Migliaccio in the 79th minute to beat Swiss second division outfit Lausanne Sport.