Siebatcheu pounced on a woeful back pass from Jesse Lingard in the fifth minute of stoppage time to spark wild scenes of celebration and ensure United got their Champions League campaign off to the worst possible start.
Robert Lewandowski scored twice as Bayern Munich opened Group E with an impressive 3-0 win in Barcelona.
Thomas Muller’s deflected effort put the Germans in front in the 34th minute. Lewandowski then scored for the 18th club game in a row in the 56th minute after the ball came back off the post.
The Poland striker grabbed his second five minutes from time when he knocked the ball in after Serge Gnabry’s shot had also hit the upright.
Juventus claimed their first win of the post-Cristiano Ronaldo era by beating Malmo 3-0 with a flurry of first-half goals.
Juventus are yet to win in Serie A this season, the last two games being losses after the departure of Ronaldo to Manchester United.
But Alex Sandro’s 23rd-minute opener was followed by two goals in the space of 60 seconds. Paulo Dybala scored from the penalty spot before and Alvaro Morata added the third on the stroke of half-time.
Juventus will be hoping they can take some of the momentum into Sunday’s Serie A clash against AC Milan.
Cristiano Ronaldo scored again for Man Utd, but they were beaten 2-1 late on by Young Boys in their Champions League opener after Aaron Wan-Bissaka’s red card.
Romelu Lukaku’s second-half header saved an under-par Chelsea as they began their Champions League defence with a 1-0 win over Zenit at Stamford Bridge.
Sevilla were held to a 1-1 draw at home to RB Salzburg after a chaotic first half in which four penalties were awarded, three to the visitors, while the hosts were forced to play most of the second half with 10 men.
Karim Adeyemi missed from the spot early one, but the Austrian’s earned a second penalty in the 21st minute, which Luka Sucic managed to tuck away.
Salzburg then won another penalty just before the break, but Sucic hit the post with his second effort. Sevilla were then awarded a penalty of their own to mark the first time in Champions League history in which four penalties were awarded in the first half.
Ivan Rakitic confidently dispatched his kick to pull Julen Lopetegui’s side level, but the hosts were soon on the back foot again early in the second half when striker Youssef En-Nesyri was sent off for a second booking.
Sevilla ‘keeper Bono made a string of saves as the Spaniards held on for a draw, but Lopetegui’s 10 men could have won the match with a late header from substitute Erik Lamela which went narrowly wide.
In Tuesday night’s other Group F game, Villarreal and Atalanta drew 2-2 at El Madrigal.
Remo Freuler gave the Italians the lead in the sixth minute, but Manuel Trigueros hauled Villarreal level shortly before half-time.
Villarreal – who beat United on penalties to lift the Europa League trophy – equalised through Arnaut Danjuma in the 73rd minute. Robin Gosens, though, struck late to earn Atalanta a point.
Dynamo Kiev saw a stoppage-time goal from Mykola Shaparenko ruled out following a VAR review as they had to settle for a point following a 0-0 draw at home against Benfica.
It finished goalless between Lille and Wolfsburg – but there was late drama after a penalty for the French side deep into stoppage-time was overturned by VAR.
Lille had earlier seen a goal for Jonathan David ruled out by VAR at the start of the second half before Wolfsburg had defender John Brooks sent off for two cautions.
Liverpool captain Jordan Henderson struck a rare Champions League goal to earn his side a 3-2 victory over AC Milan in a rip-roaring Group B opener at a rocking Anfield on Wednesday.
Henderson rifled home in the 69th minute to settle an absorbing contest between the two European heavyweights whose only two previous meetings had been in finals.
With Anfield welcoming back fans for a Champions League night for the first time in 18 months, Liverpool had threatened to run riot when Fikayo Tomori’s own goal handed them an early lead in a barnstorming start.
Remarkably, after weathering the storm which included a Mohamed Salah penalty miss, Milan struck twice in quick succession at the end of the first half with Ante Rebic and Brahim Diaz silencing the home crowd.
Milan then had a goal disallowed shortly after the break before Salah made amends for his penalty miss by equalising from close range in the 49th minute.
The outcome was still in doubt until Henderson capped a memorable Anfield night with a thumping effort — only his second in the competition and first for seven years.
“First half an hour we blew them away,” Liverpool left back Andy Robertson said. “Then we got sloppy and stopped doing the things we were doing really well. We let them back into the game and walk in at halftime thinking ‘how have we let this happen?'”
Milan’s return to Europe’s top club competition for the first time in seven years began in torrid fashion as the seven-time champions were initially overwhelmed.
With Zlatan Ibrahimovic unavailable and several players making their Champions League debut, Milan looked ill-equipped to compete and fell behind in the ninth minute when Trent Alexander-Arnold got in on the right side of the area and his cross struck former Chelsea player Tomori and beat Milan keeper Mike Maignan.
Before Milan could clear their heads they conceded a penalty when Ismael Bennacer blocked Robertson’s shot with his arm.
But Maignan dived to his right to keep out Salah’s penalty.
It was only the second time in 19 attempts that Salah had failed to convert a spot kick and it proved a turning point.
Salah fired over and Joel Matip headed straight at Maignan as the chances kept coming but Liverpool were rocked on their heels as Milan turned the game on its head — Liverpool manager Klopp saying later that his team had got a bit “carried away”.
For the first time Liverpool’s high press relented and Milan worked the ball neatly forward before Franck Kessie’s pass played in Rebic to sweep a low shot past Alisson.
Klopp wore a wry smile after that but one minute and 48 seconds later he looked as stunned as everyone else.
Milan attacked down the left again with Kessie and Rebic combining to set up Alexis Saelemaekers whose shot was blocked on the line by Robertson only for Diaz to pounce and convert.
It could have got even worse for Liverpool a minute after the restart when Saelemaekers scored from close range following a corner but an offside flag spared Liverpool.
The frantic pace continued and Liverpool were level in the 48th minute as Divock Origi, given his first action of the season, scooped a ball over Milan’s defence for Salah to dink a bouncing ball past Maignan for his 14th Champions League goal at Anfield, equalling former captain Steven Gerrard’s record.
Milan did well to soak up Liverpool pressure but they finally cracked when a corner was cleared to the edge of the area and Henderson smashed the ball home
Manchester City, last season’s Champions League runners-up, crushed RB Leipzig 6-3 in a thrilling Group A game on Wednesday as Christopher Nkunku scored a hat-trick for the German side.
It was a hugely entertaining match at the Etihad Stadium and although City manager Guardiola will be happy with the three points he will be concerned about the spaces his team left for the German side to exploit on the break.
Nkunku became only the fourth player to score a Champions League treble and end up on the losing side and he deserved his player of the match award as Leipzig showed real spirit to keep fighting back against City’s impressive firepower.
Defender Nathan Ake put City ahead in the 16th minute with a header from a Jack Grealish corner and the hosts doubled the lead 12 minutes later when Kevin De Bruyne produced wonderful skill to leave three defenders for dead before his deep cross was headed into his own net by Leipzig defender Nordi Mukiele.
Leipzig got back in the game with a 42nd-minute header from Nkunku, only for City to restore their two-goal advantage on the stroke of halftime.
Lukas Klostermann was ruled to have handled after the referee went to the pitchside monitor to check the incident and Riyad Mahrez blasted home the spot-kick.
Leipzig were not deterred, though, and Nkunku reduced the deficit again in the 51st minute by heading in a cross from the excellent Spanish midfielder Dani Olmo.
Grealish then delivered a wonderful solo goal to mark his first European club appearance, collecting the ball on the left flank and jinking inside before giving Peter Gulacsi no chance with a curling shot into the far corner.
That was surely that, the City fans could be forgiven for thinking, but once again French forward Nkunku kept the game alive, completing his hat-trick with a crisp drive after being set up by Yussuf Poulsen.
Incredibly, City responded immediately with an unstoppable long-range drive from Portuguese full back Joao Cancelo and any hopes the visitors had of a comeback ended when they went down to 10 men when former City full back Angelino was sent off in the 79th minute.
Substitute Gabriel Jesus added the sixth from close range five minutes from time and the home crowd were delighted with the display of Grealish, in front of his England manager Gareth Southgate.
“I loved it,” Grealish said of his first taste of the competition.
“I’ve been waiting a while. The past few weeks, I couldn’t wait for this game. It’s a great night.
“It was one of those games. It had absolutely everything. In the end we’re delighted to get the win against a good team full of energy and young players,” he said.
City lead the group on three points after Paris St Germain were held to a 1-1 draw at Club Brugge in their opener.