The Scottish champions' stunning 3-2 win in Moscow last month turned Group G upside down.
And Barca coach Tito Vilanova is worried his makeshift defence will struggle against Celtic's pace and aerial power. Gerard Pique joined Daniel Alves, Carles Puyol and Eric Abidal on the sidelines, while midfielder Sergio Busquets is suspended.
Vilanova said: "I am worried because Celtic are very fast up front and they proved it against Spartak. They play good, counter-attacking football so we know they are dangerous. They are also good in the air and so I am really worried about that.
"Celtic tied with Benfica and beat Spartak in Moscow which was a good thing for them.
"At the beginning, people maybe did not take into account the possibility of Celtic going through, but with four points from two games, and two games at home to go, they have a chance."
Barca have not lost a game in La Liga or the Champions League this season and sit top of Group G with maximum points after two games.
But having conceded four goals in a 5-4 win over Deportivo La Coruna at the weekend, midfielders Alex Song and Javier Mascherano look set to form a makeshift central defence again.
Vilanova tried to play down the significance of his injuries.
He said: "There are normally headaches for a coach but we have got what we have got," he said.
"All the players are at the right level to play for Barcelona.
"We would like to improve on everything – score more and concede less – and we will try to do that."
Pedro was also grilled about Barca's defensive situation but the Spain forward insists Vilanova has the squad to cope.
He said: "We have injuries, but Tito is doing well, adjusting players in defence and getting good performances.
"It is logical that it might affect us, but we have to get used to it.
"It is lucky for us that we have a great squad, but, obviously, if we have a fixed defence then it is better.
"It is a very important game in terms of us wanting to qualify.
"Celtic will wait for us and try to get us on the break, but if we win it puts us in a great situation to qualify as the best team."