Manchester United played out a 1-1 draw against AC Milan at Old Trafford.
Amad Diallo came off the bench to open the scoring for the hosts.
Kjaer restored parity for AC Milan in the last second of stoppage time to earn them a vital away goal.
Arsenal recorded a 3-1 victory over Olympiakos courtesy of goals from Martin Odegaard, Gabriel Magalhes and Mohammed Elneny to take a commanding lead into the second leg in Greece.
Tottenham defeated Dinamo Zagreb 2-0 thanks to a brace from Harry Kane.
AS Roma spanked Shaktar Donetsk 3-0 at the Stadio Olympico Stadium in Rome.
Rangers played out a 1-1 draw against Slavia Prague away from home.
Villarreal defeated Dynamo Kiev 2-0, while Ajax spanked Young Boys 3-0.
Ole Gunnar Solskjaer has hailed Amad Diallo after scoring against AC Milan.
“Perfect timing, perfect little weight on the flick. Great moment for the kid,” Solskjaer said.