Kjaer netted with a bullet header in the final seconds of stoppage time of the last 16, first leg encounter at Old Trafford, levelling the scores after Amad Diallo had opened in the 50th minute with his first United goal.
Europa League draw: Manchester United to face Zlatan Ibrahimovic-led AC Milan; Arsenal up against Olympiakos
United, who beat Real Sociedad 4-0 on aggregate in the previous round, will face the seven-time European champions in a two-legged encounter after they squeezed past Red Star Belgrade on away goals.
Europa League: Napoli, Young Boys aim for progress to Round of 32 with six qualifying places up for grabs
Here's a look at what’s happening in the final round of Europa League matches on Thursday/Friday.
Europa League: 'Loved and appreciated' Gareth Bale starts as Tottenham Hotspur win, Celtic undone by AC Milan
After coming on as a substitute against West Ham for his first appearance since returning to Tottenham, Bale played more than an hour against in the Group J match against Austria's LASK, and had a hand in Tottenham's second goal.
Champions League: Slovan Bratislava fight UEFA at CAS over qualifier forfeit under new pandemic rules
The Court of Arbitration for Sport said it was asked by Slovan for an urgent ruling to postpone the 8:15 p.m. (1815 GMT) kickoff in Switzerland between Young Boys and KI Klaksvik.
Coronavirus Outbreak: Switzerland’s federal government allows professional sports teams to resume training from 11 May
The Swiss top-tier football league was paused in February with 13 rounds left. St Gallen leads defending champion Young Boys on goal difference
European Club Association meets in Geneva over Champions League reforms as teams continue to remain divided
The battle over the future of the Champions League will resume on Monday and Tuesday when the European Club Association (ECA) meets in Geneva and the leadership faces rebellion from members.
UEFA Europa League: Arsenal drawn with Eintracht Frankfurt of Germany and 10-time Belgian champions Standard Liege in Group F
Manchester United, Europa League winners in 2017, will face Astana of Kazakhstan as well as Partizan Belgrade and former finalists AZ Alkmaar of the Netherlands in Group L.
Champions League: Red Star Belgrade, Dinamo Zagreb and Olympiacos progress to group stage of tournament
Red Star Belgrade drew their play-off second leg 1-1 at home to Young Boys in Belgrade to advance on away goals with the score all square at 3-3.
Guillaume Hoarau's second-half penalty claimed a 2-2 draw to deny former winners Red Star Belgrade a comeback victory over Young Boys.
Champions League: Nothing surprised me in Manchester United's loss to Valencia, says manager Jose Mourinho
Defeat was nothing more than Mourinho's side deserved, as a line-up that included eight changes and a return for Paul Pogba delivered a sluggish display at Mestalla.
Champions League: Real Madrid, Manchester United pay price for fielding weakened teams; Bayern, Ajax play six-goal thriller
Real Madrid and Manchester United paid the price for fielding weakened teams on the final night of Champions League group play, with Madrid slumping to a record home loss and United squandering a surprise opportunity to top their group.
Champions League: Cristiano Ronaldo won't be rested for Young Boys clash, says Juventus coach Massimiliano Allegri
Cristiano is the record scorer in the elite European competition with 120 goals and scored 15 for Real Madrid on their way to their third title last season.
Champions League: Marouane Fellaini says Manchester United players are upbeat despite recent struggles
Marouane Fellaini said the mood in the Manchester United camp is upbeat despite their recent struggles after he scored a late winner to secure qualification for the Champions League knockouts.
Champions League: Jose Mourinho tells critics to 'come and sit on the manager's bench' after scrappy win over Young Boys
Jose Mourinho once again stole the show from his players, smashing drinks crates to the ground after Marouane Fellaini's late winner took Manchester United into the Champions League knockout stages and eliciting headlines on Wednesday such as "Bottle of Red".
Premier League: Jose Mourinho says Diogo Dalot will be eased into first team after impressive debut against Young Boys
Jose Mourinho declared that Diogo Dalot, 19, was the best young full-back in Europe after signing him from Porto for £19 million in June.
Champions League: Manchester United boss Jose Mourinho upbeat after Paul Pogba shines against Young Boys
With Paul Pogba putting his stamp on games and Jose Mourinho in an upbeat mood, the atmosphere around Manchester United is markedly more relaxed than it was just a few weeks ago
Champions League: Cristiano Ronaldo sent off in Juventus' victory over Valencia; Manchester City stunned
A tearful Ronaldo left the pitch in disbelief in the 29th minute, having aimed a petulant, but seemingly harmless, kick at Valencia defender Jeison Murillo. Ronaldo then made contact with Murillo's head while urging him to get up.