Manchester United allowed a late away goal to draw 1-1 with AC Milan in their Europa League last-16 first-leg tie at Old Trafford.
The 18-year-old Diallo, who joined the club from Atalanta in January, had made headlines with his displays with the United Under-23 side and made an immediate impression after coming on as a second-half substitute on Thursday, rising to guide a well-placed header out of the reach of Gianluigi Donnarumma in the 50th minute.
The tally was the Ivorian’s first attempt at goal for United and made him the youngest non-British player to ever score in major European competition for the club.
United, who saw an 11th-minute Franck Kessie goal ruled out for handball by a VAR review, looked set to survive the match without allowing a critical away goal, but Kjaer lost his marker and powered his stoppage-time header hard and past Dean Henderson to give Milan a slight advantage heading into the second leg.
The two sides will play the return match next Thursday at the San Siro.