Neil Lennon has admitted Celtic's Champions League dream is hanging by a thread after a disappointing 1-0 defeat at Ajax.
The loss in Amsterdam leaves the Hoops bottom of Group H with just three points from four games.
Celtic now need to beat Milan at home and may need to get something from their trip to Barcelona to stay involved on the continent.
Lasse Schone, who had scored after coming on as substitute in the 2-1 defeat at Celtic Park, grabbed the winner with a well-taken goal in the 51st minute.
Former Northern Ireland international Lennon (pictured) said: "It is imperative that we beat Milan at Celtic Park if we are to have any chance of qualification.
"But we will have to play better than on the first-half evidence tonight.
"Our play in general was not like us tonight.
"We were very flat in the first half and I think some players will have regrets because they were nowhere near their best and at this level you can't have that."
AC Milan visit Celtic Park later this month before the Hoops travel to the Nou Camp in December.
The game was marred by a clash between supporters and police in the centre of Amsterdam before kick-off, following which 15 fans have been arrested. Amsterdam Police said that fans armed with bottles and sticks attacked plain-clothed police.
Meanwhile, Aaron Ramsey's header gave Arsenal a battling victory over Borussia Dortmund that keeps them at the top of Group F.