They say life begins at 40, and for Ruben Amorim the first game of his fifth decade was one of the more enjoyable of his Manchester United tenure so far.

The day before his milestone birthday on Monday, he joked that after two months at Old Trafford, he felt more like a 50-year-old than a 40-year-old. This has become a demanding hot seat that ages everyone who sits in it, and Amorim was feeling the strain.

It felt like an omen that right next to FCSB's impressive National Arena was a giant building with a neon sign offering cardiovascular treatment. Coaching United is a stressful experience, watching them equally so. You worried that they would be queuing out of the door by midnight.

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But on a night that could have been tricky, they turned in one of the more complete performances of the Amorim era. Piece-by-piece, he is beginning to find his best side, and in Bucharest, the jigsaw looks a little more complete. When the teamsheets landed, everyone expected Kobbie Mainoo to sit deep and Bruno Fernandes to play in front of him.

When the game kicked off, those roles were reversed, and as both players warmed to the task, they began to make United tick. With Lisandro Martinez growing in confidence and Mainoo and Fernandes dovetailing on the left side of midfield, that area of the pitch became crucial.

Mainoo has struggled to nail down a place under Amorim, and this window has opened the door to the possibility of the midfielder leaving. It remains plausible that Alejandro Garnacho could yet depart on Monday, but with the Argentine again producing an impressive second-half cameo, there must surely be a demand to find the money to reinvest elsewhere.

This game had more riding on it for the hosts, who began the night sitting in the last automatic qualification spot. Their excellent support upped the occasion, unfurling a stunning tribute to former goalkeeper Helmut Duckadam pre-match, the man who saved four penalties against Barcelona in the 1986 European Cup final.

That was when this club were still known as Steaua Bucharest and a reminder of former glories. Those days are long gone for clubs in the Eastern bloc now, given the power and financial imbalance to western Europe, although United are living proof that money isn't always the answer.

Amorim is trying to rebuild this club and the culture around it. His pre-match press conference had again centred on a player who wasn't in Romania rather than any of the 21 who were. With Marcus Rashford still banished, Amorim planned rotation and was true to his word, although his changes had to be limited. Last week's dramatic win against Rangers all but secured a top-eight finish, but there was a mathematical miracle that would have dumped United back into the play-offs had they failed to win here.

That would have been disastrous. Amorim has clarified how important those two free midweeks in February would be. He sees training ground time as the quick fix to his problems, and those weeks could now be valuable to continue the development of this team.

The top-eight finish also secures a more comfortable draw when the knockout action begins for United in March. With their Premier League campaign becoming a nightmare, winning the Europa League is the only route back to the Champions League and the riches on offer. The seven games standing between United and potentially lifting this trophy in Bilbao could be their most important of the season.

By the time the competition resumes, this side could have taken another step forward and Mainoo may be more integral than he has been to date. He was proactive in trying to stretch FCSB and run in behind from the early stages, often getting space and offering an out-ball to Martinez.

He should have scored early on, with Rasmus Hojlund this time occupying the channel on the left and pulling a ball back to an unmarked Mainoo, who blazed over. Hojlund himself then sent a header straight at goalkeeper Stefan Tarnovanu, who also pushed out a Christian Eriksen shot.

United were comfortable against a limited side, and they gradually began to find a more flowing game in the final third. Garnacho, one of two half-time substitutions, was presented with one of the better chances via a poor back pass, but his shot hit the outside of the post.

Hojlund again forced a routine save from Tarnovanu when he tried his luck from an angle, having been well found by the excellent Martinez before Fernandes rattled the crossbar with a strike from distance.

The breakthrough felt like it was coming and it was a well-worked goal from a throw, an area assistant coach Carlos Fernandes has been looking at. Eriksen's header found Mainoo in space, and his low cross was a gift for Dalot to turn home.

FCSB had barely threatened until then, but with the goal dropping them out of the top eight, they had to try and respond, and they came close to doing so when Daniel Birligea smashed a 25-yard shot off the underside of the bar and out.

Their fate was sealed when the outstanding Mainoo killed the game off. After FCSB had lost possession, Mainoo found an angle to play a pass to Garnacho. The winger returned the favour and Mainoo produced an excellent low finish.

Garnacho then produced fancy footwork and a neat backheel to tee up Fernandes, who clipped a shot wide, but for United the job was done. They have the February they want.