AC Milan turned up the heat further on Neil Lennon by recording a 3-1 victory over Celtic in their opening Europa League encounter.
Lennon and his players came into the game with their backs against the wall after the 2-0 defeat to Rangers on Saturday and started poorly as Zlatan Ibrahimovic - at 39-years-old - ran the show at Celtic Park.
Rade Krunic headed the Italians in front (14) before Brahim Diaz doubled the lead with a neat low finish (42).
Celtic improved after the break when switching to a back four and Mohamed Elyounoussi gave Lennon's team hope (75) but they were unable to fashion an equaliser. Jens Hauge wrapped up the points in stoppage time as AC Milan stretched their unbeaten run in all competitions to 21 matches.
Celtic: Barkas (6), Welsh (5), Duffy (4), Ajer (6), Frimpong (7), Ntcham (6), Brown (5), McGregor (5), Laxalt (7), Ajeti (6), Griffiths (6)
Subs: Elyounoussi (7), Rogic (7), Christie (7), Taylor (6)
AC Milan: Donnarumma (7), Dalot (6), Kjaer (7), Romagnoli (7), Hernandez (7), Krunic (7), Tonali (6), Kessie (7), Castillejo (7), Ibrahimovic (7), Diaz (7)
Subs: Bennacer (6), Hauge (7), Leao (6)
Man of the match: Zlatan Ibrahimovic
How AC Milan made a bad week worse for Celtic...
Once again there were no fans allowed inside the stadium but Celtic started as if they were being cheered on by a packed house.
However, they were hit by an Italian sucker punch.
Leigh Griffiths was handed a start by Neil Lennon after coming off the bench to good effect against Rangers.
Meanwhile, Zlatan Ibrahimovic provided plenty of intrigue in an AC Milan starting XI, that incuding Sandro Tonali, who replaced Ismael Bennacer.
Castillejo drifted in from the right and flighted a cross in for Krunic to get in between defenders Stephen Welsh and Shane Duffy to direct his header past Vasilis Barkas.
Celtic kept up a good tempo as they fought back but then the Serie A side began to occasionally flex their muscles and by the interval, they had doubled their lead.
In the 42nd minute Ibrahimovic sent defender Theo Hernandez racing down the left and when he passed inside to Diaz, the attacker neatly side-stepped Duffy and Callum McGregor and slotted a shot low past Barkas.
Lennon replaced Welsh and Leigh Griffiths with Elyounoussi and Ryan Christie for the start of the second half and went to a back four in a reshaped formation with some early joy.
In the 51st minute, Celtic wing-back Diego Laxalt, on loan from the visitors, ended a promising move by driving just over the bar from the edge of the box.
Tom Rogic replaced skipper Scott Brown in the 64th minute but it was another replacement, Elyounoussi, who gave Celtic a lifeline when he headed in from Christie's delivery.
The home side gave it their all in search of a leveller but in the second minute of three added, Milan broke and Alexis Saelemaekers played in Hauge and his fellow substitute slipped the ball past Barkas.
Man of the match: Zlatan Ibrahimovic
Ibrahimovic became the oldest outfield player to start a UEFA European match for AC Milan since Paolo Maldini, who was 40 years old when featuring against Weder Bremen in 2006. And he proved that age really is only a number with a commanding performance, one that made his team tick from his role roaming free in the hole. His awareness of players around him still is world-class as is his ability to pick a pass as shown by his ball in behind to kickstart the attack from which Dias doubled AC Milan's lead.
What the manager said
Celtic boss Neil Lennon: "I'm disappointed we lost. I thought we deserved something from the game. We were very good in the second half, a lot better, better play, better body language but we lacked a cutting edge.
"We got into good areas in the first half but the final ball needs to be better at this level, we gave away two soft goals.
"But we made changes at half-time and we played better in the second half.
"It was markedly better than the weekend. It was encouraging. I don't like losing but there were a lot of good signs for me.
"I wanted an improvement and we got a big improvement. Part of me is quite pleased with that.
"We could have shied away from it all after the criticism we have taken and I have taken but they showed their personality in the second half."
- AC Milan are unbeaten in their first UEFA European match in each of their last 18 seasons (W15 D3) since losing 3-2 to FC Porto in the 1996/97 Champions League.
- Celtic have lost each of their last four UEFA European matches against AC Milan, losing three times in the Champions League along with tonight's defeat.
- Krunic scored his first goal for AC Milan in all competitions in his 25th appearance for the club. He also became the first Bosnian player to score a European goal for Milan.
Celtic have a tricky trip to Aberdeen to negotiate on Sunday, a game live on Sky Sports Football at 12pm. They then play Lille next Thursday in their second Europa League fixture.