In a statement, Milan said that Loftus-Cheek "will be re-assessed in about a week due to the recurrence of a hamstring problem in his right thigh".
England international midfielder Loftus-Cheek has only made a substitute appearance in Milan's thrilling Super Cup final win over Inter in the last month.
Milan host Cagliari on Saturday before travelling to Como on Tuesday, the latter fixture one of the two games that Sergio Conceicao's new team have in hand.
Loftus-Cheek is also at risk of missing Milan's clash with rivals Juventus on February 18th, with the Champions League visit of Girona coming the following Wednesday.
Milan are eighth in Serie A, 17 points behind leaders Napoli, but have a good chance of direct qualification for the last 16 of the Champions League as they are a point from the top eight with two league phase matches remaining.