Forrest missed the Scottish Premiership win over Hearts at Tynecastle on Saturday with an ongoing sciatica problem but has also been taken ill and was absent when the Hoops squad flew out of Glasgow Airport to Italy.
Midfielder Joe Ledley has a groin problem but checked in with the rest of the Hoops squad as did fellow Wales international Adam Matthews, who hobbled off against the Jambos with an ankle injury.
New signing Nir Biton will meet up with his new team-mates in Milan but the Israel midfielder is unlikely to feature.
Celtic assistant Johan Mjallby said: "Unfortunately James is ill. He has been feeling that sciatica problem lately but at the same time he is ill.
"It is brilliant that Joe is here. When I left the training ground he was struggling and we were pretty sure that he would not be able to make it so that is great news.
"The scan has come back and obviously shown that he has a chance."
Milan boss Massimiliano Allegri has also injury problems to contend with as up to five first-team starters, including skipper Riccardo Montolivo and new signing Kaka doubtful.
Allegri also has four other definite absentees leaving the Rossoneri up to nine men short.
However, Mjallby said: "I am not going to be fooled by that. AC Milan is always going to be a very difficult task.
"They have a lot of quality players, they are a strong team and we have to be at our best to get something from the game"
You can watch Milan v Celtic live on Sky Sports 2HD at 7.30pm