On 68 minutes Kobbie Mainoo and Alejandro Garnacho combined for the Manchester United second that sealed passage into the last 16: over went a delivery from the No 17 and there was Mainoo to finish coolly. For two young talents who are on the market for the right price here was a satisfying moment that showed Ruben Amorim what he can harness if each are retained beyond Monday’s deadline. It secured, too, a third consecutive win for Amorim, the first time United have done so this season, and gives him a February clear of a playoff to concentrate on training: something the head coach has referenced.

Mainoo’s strike followed Diogo Dalot’s opener, which also came from United splaying the FCSB backline – one more positive Amorim can build on. Yet the Portuguese will wonder what version Crystal Palace encounter on visiting on Sunday: Amorim is fully aware his United remain an enigma. This began as a contest of the £32m-costing Romanian champions taking on the £267.5m visitors, £72m of which was Rasmus Højlund, the United No 9 who had failed to score in 23 of 28 games this term.

Part of the Dane’s problem is a lack of service, as when Christian Eriksen floated in a free-kick, the No 9 leaped to head, but the ball was aimed at Matthijs de Ligt, who connected weakly. Amorim sent out a United XI containing the fringe talents of Toby Collyer, Altay Bayındır and Tyrell Malacia. When the latter scythed down Florin Tanaes, FCSB’s central forward, he was booked, and Risto Radunovic’s dead ball went close to punishing the left wing-back’s recklessness. Here again, United’s powderpuff backline failed.

The No 33’s delivery was a diagonal Noussair Mazraoui, De Ligt and Lisandro Martínez all missed and they were thankful Mihai Popescu’s sliding volley did the same, going wide of Bayindir’s left upright. Here, a glimpse for FCSB, who are second in their league, that a famous win was more than possible. As they should, United retained a dash of potency, certainly for bisecting their hosts as when Højlund ran wide to turn back Martínez’s pass in behind. The Dane, twisting, found Kobbie Mainoo ghosting through but he blasted over from point-blank range.

Earlier, Mainoo had also schemed on to a Bruno Fernandes pass but again the finish disappointed, spooned into Stefan Tarnovanu’s gloves. The 1,500 or so United enthusiasts who had made the trip could be happy, though, as at this juncture those in white spun the ball about at a pace pleasing to Amorim, Eriksen warmed Tarnovanu’s fingers with a bullet, and when Steaua had possession, United pressed quickly.

But Højlund’s toothlessness was evident when a free header, near-in, from Martínez’s flighted ball did not trouble the keeper. This, or one of Mainoo’s two chances, had to be taken as United are no goal machine, and as this opening 45 minus aged, those in blue edged back as a force, pinning their visitors back near their area. United’s shot count was six to the Romanian’s paltry two. Their possession percentage was 69. At the interval they were in the fourth place they began the night, Rangers, two points back, the closest to stymieing their bid to avoid the playoff round. To shake up proceedings Amorim’s options were to stick or make a change – Amad Diallo’s zest and Alejandro Garnacho’s directness, his best options for this, as these were the head coach’s sole attackers on the bench.

Sign up to Football Daily. Kick off your evenings with the Guardian's take on the world of football. after newsletter promotion. Manchester United and Tottenham cannot meet until the Europa League final after finishing third and fourth in the league phase table. The two Premier League clubs will be placed on different sides of the bracket in Friday's draw, and will both play winners of the playoffs between AZ or Midtjylland and Real Sociedad or Galatasaray.

Rangers' final-day climb into the top eight means they skip the playoffs, but they could face the teams they pipped to eighth place – Bodø/Glimt or Anderlecht – while their potential playoff opponents, Twente and Fenerbahce, also look dangerous after just avoiding elimination. If Rangers advance to the quarter-finals, they are likely to face either Lazio or Athletic Bilbao, although the group-topping Serie A side could play arch-rivals Roma in the last 16. Manchester United and Tottenham will be in a quarter with Eintracht Frankfurt or Lyon, depending on the draw's outcome.

This was precisely what the Portuguese plumped for. Off went Malacia and Collyer, and on came the Ivorian and Argentinian. The latter’s first touch was to clip Tarnovanu’s left post, Garnacho latching on to a poor back pass: the speedster raced forward but, in a microcosm of a chief United ill, his radar was awry. Eriksen seemed to favour De Ligt as the far more likely to net than an actual forward. You could hardly blame him. At a second free-kick one of the two Amorim No 10s pinged the ball over from the left, it bounced on to the Dutchman ,but he mirrored Mainoo, Garnacho, Eriksen and Højlund, in his pale imitation of a lethal marksman.