There has been "some progress" in talks between Celtic and the banned Green Brigade supporters group, according to interim manager Martin O'Neill.
An initial six-game ban was imposed on around 200 people in early November following "violent and threatening behaviour" at a home match against Falkirk on 29 October, before the ban was extended at the start of December due to "safety incidents".
O'Neill says certain elements of the Parkhead atmosphere have been "lost" and again reiterated the need for "unity" between fans and the club's hierarchy.
"A full house here at Celtic Park is something special," O'Neill said when asked about the Green Brigade's absence.
"In every European night that I've had in the past, teams, and I'm talking about the really major sides, were still very concerned about coming here with the passion, the drive and the noise that's made.
"I remember here at our game against Liverpool in that Uefa Cup run, John Robertson, my old friend, was standing beside me and speaking to me and I couldn't hear a word he was saying for the first 10 or 15 minutes. It was so, so loud.
"So, yeah, little bits have been lost from that and that is, that's regrettable, to use your word.
"I think there has been some discussions, which I'm obviously wasn't party to, but I I think that there's been some progress made.
"But as I've been saying this here, the sooner, the sooner that there is unity at Celtic Park, the better, the better for all concerned.
"I have no doubt at all that the crowd behind the goal on Sunday contributed greatly to us fighting back in the game. No question about that."