John Sivebaek said: “It was a surprise he did not start against AC Milan.
“I have no explanation for it. As a player you have to accept the manager’s decision and that is what Thomas is doing.
“We all know, though, a player at his level cannot accept the situation as it is for a long time. Thomas is still happy at Celtic, and he has two and a half years remaining, but things can change quickly.”
David Beckham will join US Major League outfit LA Galaxy this summer, but they have also expressed their interest in the Danish midfielder.
Galaxy head coach Frank Yallop said: “Thomas Gravesen would be welcome here. He is a very good player with great experience at the highest level of club and international football. I think he could be a superstar in the MLS.”