Simon Kjaer grabbed a last-gasp equaliser to give AC Milan a dramatic 1-1 draw at Manchester United after Amad Diallo scored his first goal for the hosts in the Europa League last 16 first leg on Thursday.

Kjaer netted with a bullet header in the final seconds of stoppage time at Old Trafford.

The Milan defender's late intervention spoiled a memorable moment for Ivorian teenager Diallo.

In only his third appearance for United, Diallo gave a glimpse of the potential that persuaded the Premier League club to pay £18 million ($25 million) to sign him from Italian side Atalanta in January.

Just five minutes after coming on as a half-time substitute, the 18-year-old put United in front with a superb header.

Legends including Paolo Maldini, Cristiano Ronaldo, Kaka, Wayne Rooney and Ronaldinho have all featured in previous clashes between United and Milan.

While it is far too soon to place Diallo in that company, United manager Ole Gunnar Solskjaer will hope that scoring with his first ever effort on goal bodes well for the much-hyped youngster's future with the club.

At 18 years and 243 days, Diallo is the fourth youngest player to score for United in a major European competition, after Mason Greenwood, Marcus Rashford and George Best.

"Perfect timing, perfect little weight on the flick. Great moment for the kid," Solskjaer said.

While Diallo's headline-grabbing act was significant, it wasn't enough to help United build on the momentum from their 2-0 win at Premier League leaders Manchester City on Sunday.

With United 14 points behind City, the Europa League and FA Cup represent their best hopes of winning the first trophy of Solskjaer's reign.

"You're always disappointed when you concede a goal in that manner," Solskjaer said.

"One minute before full time is also disappointing but we've got to take that on the chin and just travel down there next week and go for it.

"We know we have to score a goal and win the game now, or score two and draw. It makes it more difficult."

The second leg in Italy next Thursday won't be easy for United on the evidence of a composed and determined performance from injury-hit Milan's crop of promising youngsters.

"We are working to grow and bring Milan back where we belong," boss Stefano Pioli said.

"It was a great performance and a great result, but we need another perfect performance to qualify against such a strong team.

"I keep repeating to my players that I'm not coaching a normal group. This an extraordinary group, the lads always give their 100 per cent in every game, they are just incredible."

In a sign of the drop in standards at Old Trafford and the San Siro, the first meeting between two of the world's most historic teams since 2010 was their first outside the Champions League knockout stages.

Both hope they are gradually on the way back to the top.

Pioli's side were denied an 11th-minute opener after Franck Kessie pounced on a long throw and smashed a superb strike past Dean Henderson from just inside the area.

Kessie's celebrations were premature as a VAR review showed the ball had glanced off the midfielder's arm before his shot.

Harry Maguire was guilty of an incredible miss when the United defender, no more than a yard out, somehow shot against the post after Bruno Fernandes flicked on Alex Telles' corner.

United had moved the ball too slowly and with too little purpose in the first half, so Solskjaer replaced Anthony Martial with Diallo at half-time.

Diallo's introduction proved an inspired move as he broke the deadlock in the 50th minute.

Fernandes unfurled a delightful long pass towards Diallo, who made an incisive run behind the Milan defence and looped a clever backwards header over Gianluigi Donnarumma.

But United failed to build on that lead and Milan's intelligent play was rewarded with just seconds left when Kjaer powered his header past Henderson from a corner.