Gravesen joined Celtic for £2million from Real Madrid last summer.
Sivebaek told the Scottish Sun: “The only reason that Thomas has stayed with Celtic is because of how much he loves the club.
“He thinks the club is fantastic and the fans are fantastic.
“The only thing missing is he’s not playing football. He’s not getting a chance and it’s my personal opinion he is being frozen out.
“He’s frustrated right now, but he accepts the situation, even if he does find it very strange.
“If, come the next transfer window, he’s not playing, we’d have to consider our options.”
