The 27-year-old was not even picked in the squad to travel to Spain in midweek.
The former AC Milan man played in the reserves' defeat at Hamilton on Monday and his agent Andrea D'Amico has spoken of the midfielder's frustration.
"We want to speak to Celtic," said D'Amico in the Evening Times.
"Massimo is unhappy, he is not playing and was left out of the Champions League squad for Villarreal.
"He cannot understand what he has done wrong. It's strange, the manager explained to Massimo in the summer that Celtic wanted him."