An early goal from Kaka was enough to give AC Milan a half-time lead away to Celtic as they look to progress to the Champions League knockout stages.
The Brazilian rose unchallenged at the edge of the six-yard box to beat Fraser Forster and put the Italian side in front.
Mario Balotelli may consider himself unfortunate not to have doubled the Rossonieri's lead six minutes before the break, when his run on goal was flagged for offside despite replays suggesting Mikael Lustig was playing him onside.
Giorgios Samaras arguably came closest just before the interval when he rose above Daniele Bonera, but his header was comfortably dealt with by Christian Abbiati in the visitors' goal.
Should Ajax win and Celtic fail to find a way back into the game, the Scottish champions would be out of Europe altogether.
