The 22-year-old winger will be asked how he is feeling after missing out at the weekend with a recurrence of his sciatic nerve problem while on international duty with Scotland.
But while Neil Lennon considers he is a very important part of his plans for winning in the San Siro, the Hoops boss is concerned Forrest's eagerness to play in big games is clouding his own judgment.
Lennon has explained to the youngster that is helping no one - especially the player himself.
Asked if Forrest had to train today to prove his fitness for inclusion in the squad for the opening Champions League Group H match, Lennon answered unequivocally: "No, I will be guided by the physios.
"James has got to be honest with us about how good he is feeling.
"We have told him he is not doing us any good if he is not honest, but, more important, he is not doing himself any favours."
Forrest's misfortune with injury has already cost him the chance to play in many of the club's biggest games since he broke through to the first team three-and-a-half- years ago.
So, Lennon can understand why he is prepared to say he is ready to play when he is not fully fit.
But the manager believes the time has arrived for cards to be laid on the table, rather than gambles to be taken.
Lennon said: "James wants to play. But if he is only 80% fit, there is a drop (in his form).
"When he is 100% fit, he is such an asset for us - and he knows that."
Lennon will also speak to the club's physios about Adam Matthews, to check on his recovery from the ankle injury which forced him off during the 3-1 win over Hearts.
But early indications are the Welshman will be okay to travel with the squad tomorrow.
Lennon watched on as Milan's own injury problems increased on Saturday night while they were held to a 2-2 draw by Torino in Turin.
Coach, Massimiliano Allegori, confirmed: "Ricardo Montolivo, Mattio de Scilgio, Ignazio Abate and Stephan El Shaarawy will not play on Wednesday.
"We will make do with who we have and we will have fresh players ready to play what is a delicate and important game."
One of the men expected to be given his chance against Celtic is midfielder, Valter Birsa, who joined the club from Genoa in the summer.
The 27-year-old Slovenian said: "It will be an intense match against Celtic and we have to win to bring home the points and hope everything goes well.
"Celtic are a physical team, very fast and with high balls they can hurt you. But the environment at Milan is completely different than the other teams. There is quality in everything."
Another of Celtic's Group H opponents, Barcelona, have also been dealt an injury blow with the news that full-back, Jordi Alba, will miss the visit to Parkhead on October 1 with a torn hamstring.