"We could play, but we have to wait until after training shows how it feels, it shows positive things in training, but we need a certain safety threshold to be able to play," said Valverde.
"I could play at the beginning, go with the bench, I would not even play … everything is possible. What's certain is that we will not risk it."
Messi broke his arm between Barca's victory over Sevilla on October 20, he estimates the club when he was out for three weeks.
However, the 31-year-old Argentine with San Jose has traveled the team and can still be proclaimed.
"He's glad he's with us, and we're happy to have him here in Milan. It's always good that we have Leo with us," added Spanish defender Sergi Roberto.
"We organized in his absence, without him we won, but we are glad to have him with the group."
Barcelona won all of their last four matches without Messi, including a 2-0 win in Inter in Camp Nou last month. They comprise three points ahead of the Italians at the top of group B, who won each of the three matches.